The following is entirely speculation from the dungeon journal and the very brief snapshots we have seen of the boss room. It is likely mostly wrong, but also mostly right. You should walk away from this more confused than when you began.

The fight against Gallywix is a multi-stage, highly technical encounter that will stress your raid’s ability to coordinate and move as a unit throughout. It revolves around sabotaging the Giga Coils around the encounter space by using the Giga bombs dropped by adds you can defeat during the fight and dealing with the various Cannisters used. By using Gallywix’s inventions against him, you can turn the tide and defeat him, putting an end to the Chrome King’s reign and stopping him from unleashing his twisted weaponry against his own people.

Full List of Boss Abilities for Reference

Scatterblast Cannisters
Mechengineer’s Cannisters
Cannister Detonation
Big Bad Buncha Bombs
Blast Burns
Bad Belated Boom
Sapper’s Satchel
1500-Pound “Dud”
Zagging Zizzler
Suppression
Focused Detonation
Time-Release Crackle
Another in the Bag
Suppression
Venting Heat
Overheating
Gatling Cannon
Trick Shots
Giga Coils
Giga Blast
Giga BOOM!
Sabotaged Controls
Control Meltdown
Overloaded Bolts
Fused Cannisters
Charged Giga Bomb
Fling Giga Bomb
Giga Bomb Detonation
Juice it!
Party Crashing Rocket
Shock Barrage
Wrench
Lumbering Rage
Party Crashing Rocket
Cratering
Armageddon-Class Plating
Radiant Electricity
Total Destruction
Overloaded Rockets
Bigger Badder Bomb Blast
Ego Check
Checked Ego

Important Abilities

As you can tell, there’s a lot of abilities in this fight. We’ll break it down by each phase.

Phase One

Scatterblast Cannisters - Gallywix fires a blast at his current tank, inflicting a massive amount of damage to them. This portion cannot be dodged or parried. Additionally, this also blasts players in a frontal cone centered on the tank (presumably moving with them), dealing a massive amount of damage (28M on 10H, scaling) but split between all players struck. This also increases their damage taken from this ability by 100% for 30 seconds. All targets hit become afflicted with Mechengineer’s Cannisters. If this fails to hit at least 3 players, this will instead hit all players for the full, unsplit damage, resulting in a wipe.
Mechengineer’s Cannisters - A healing absorb placed on those who soaked the Scatterblast Cannister, which lasts for 30 seconds. Upon expiration or being fully consumed, this will deal damage to the player based on how much was left in the absorb. On Heroic, this deals damage to the entire raid based on how much was left. Presumably, if it’s consumed, it will deal no damage. If it is not healed whatsoever, it does the full amount of the absorb to all players for each person not healed.
Big Bad Buncha Bombs - This is one ability, but has several components to it. The first is the main ordnance, which is a rain of artillery that deals lethal damage to any players caught in the indicated visuals. Any players hit by any of these components receive Blast Burns, a stacking, light damage but long-lasting (45 seconds) DoT.
Bad Belated Boom - The next component of Big Bad Buncha Bombs. Several of the bombs had out-of-compliance safety fuses, causing them to detonate late. These inflict moderate damage to players within the indicated 7-yard radius, and applies Blast Burns.
Sapper’s Satchels - The third component of Big Bad Buncha Bombs. Several players will be marked, exploding after 5 seconds for light damage. Any players within 7 yards when these detonate receive that damage, Blast Burns AND Sapper’s Satchels. On Heroic, when the marked players detonate, they also drop a pile of bombs at their feet, which explode moments later dealing moderate damage.
1500-Pound DUD - On Heroic, an unknown amount of munitions will embed themselves into the arena, exploding after 15 seconds for high damage to the entire raid and applying a high damage DoT that stacks. A player can instead interact with this ordnance to focus the blast, similar to Z’skarn, instead taking 10 Million damage and knocking them back. The player who triggers this will also receive Focused Detonation, a high damage DoT that lasts 10 seconds. Lastly, shortly after Focused Detonation expires, a Time-Release Crackle blasts throughout the arena, dealing light damage to the raid.
Zagging Zizzlers - Miniature rockets fly about the various piles, dealing light damage to the raid for 30 seconds. This stacks.
Suppression - Gallywix shrapnels players in indicated locations for 3 seconds, inflicting light damage on impact and then afflicting players hit with a light DoT that stacks every 0.5 seconds they remain in the location.
Venting Heat - Gallywix’s machines dump excess heat, burning all players in the arena, dealing a light burst of damage up-front then very light damage over 4 seconds. Every time he casts this, the next cast is 1 second longer due to the inefficiency of his invention’s heat dissipation.
Gatling Cannon - Instead of melee’ing, Gallywix shoots his main target, dealing up to 200% increased damage based on how far they are from him.
Trick Shots - Gallywix gains this buff every 4 seconds, stacking. If he reaches 10 stacks of this OR if his main target changes, he will release all stacks, dealing 2.5 Million physical damage plus a ~400k a second DoT per stack that lasts 9 seconds. This cannot be dodged.

Phase Two

Gallywix retains much of the abilities from Phase One, like Big Bad Buncha Bombs and all related components, Suppression, and Venting Heat, but Scatterblast Cannisters has been modified, and he now summons additional adds and makes use of the Giga Coils that surround the arena.

Giga Coils - Gallywix charges and activates one of the Giga Coils around the room, causing moderate ticking damage every second. On Heroic, this also causes Overloaded Bolts, creating swirlies to dodge that cause light damage in indicated locations.
Giga Blast - Gallywix collects the power of the Giga Coils and ejects it in a direction, several times, dealing moderate damage to anyone struck and also afflicting them with a light, stacking DoT. Each successive blast increases Giga Coil damage by 15%.
Giga BOOM! - Giga Bombs detonate after 30 seconds or when struck by Giga Blast, inflicting moderate damage to the raid plus a light DoT that ticks every second for 9 seconds. This stacks per explosion.
Sabotaged Controls - When both Giga Controls on a Giga Coil are damaged, they spark occasionally, dealing light damage to targeted players every 6 seconds per each Sabotaged Control.
Control Meltdown - Sabotaging a Giga Controls causes this for 1 minute. If Gallywix attempts to cast Giga Coils during this time, it backfires, interrupting the cast.
Sabotage Zone - When both Giga Controls on a Giga Coil are damaged, this covers their quadrant of the room, dealing moderate damage every second to players in the area.
Fused Cannisters - Gallywix marks several players and launches shells at them that detonate after 6 seconds. Upon detonation, this deals light damage, split evenly among everyone in an 8 yard radius. All targets who soak this receive Mechengineer’s Cannisters, the same effect from Phase One and working similarly, exploding for damage to the player equal to the healing absorb remaining. On Heroic, this deals damage to the whole raid instead, just as in Phase One. Any marked player who fails to share this effect with at least 1 other player causes this to instead affect the entire raid for the full effect.
Darkfuse Technician - These arrive in on a Party Crashing Rocket, dealing light damage to any players within the indicated visual on impact. These do not do much, except Juice It!, which charges a Giga Bomb, causing it to explode for light damage in a 10 yard radius and increase their damage and movement speed by 5%, stacking. These drop Charged Giga Bombs on death.
Giga Bomb - These are dropped by Darkfuse Technicians on death, and radiate very light damage in a 3 yard radius. They can be picked up by players, instead focusing that damage to them. A player carrying this Bomb can choose to Fling Giga Bomb, throwing it to a targeted location. Carrying this near a Giga Controls causes Giga Bomb Detonation, inflicting a jolt to the player carrying the Bomb and sabotaging the Controls. This jolt also gives the player 30% movement speed for 10 seconds.
Sharpshot Sentry - These cast Shock Barrage at a random player, an interruptible channel that deals high damage over 4 seconds.
Darkfuse Wrenchmonger - These arrive in on a Party Crashing Rocket, dealing light damage to any players within the indicated visual on impact. These can be tanked and deal high damage with Wrench, which debuffs their main target with every cast, increasing their damage taken from Wrench by 40% a stack. They also enrage with Lumbering Rage, gaining 50% damage but 50% reduced movement speed. Intermission
Cratering - Gallywix slams down into the arena, dealing massive (and certainly lethal) damage to any players within 20 yards of the impact, and moderate damage to the entire raid.
Armageddon-Class Plating - A shield from the Darkfuse that prevents Gallywix from being interrupted.
Radiant Electricity - On Heroic, Gallywix rotates a beam of lightning that stuns and deals moderate damage to any player caught.
Total Destruction - Gallywix rains down rockets on the entire raid, causing light damage that increases by 20% with every tick, lasting 30 seconds.

Phase Three

Gallywix retains a mix of abilities from Phase Two, with some modifications to make them more deadly. He no longer summons Sharpshot Sentries or Darkfuse Wrenchmongers, however. Fused Cannisters have become Tick-Tock Cannisters. Several abilities from Big Bad Buncha Bombs have also seen modification, and Suppression has a new effect.

Big Badder Bomb Blast - Gallywix shells his primary target with this, inflicting moderate damage to them anyone with 30 yards. This also causes anyone hit to take 200% increased damage for 1.5 minutes. Anyone hit also receives Blast Burns.
Overloaded Rockets - Sapper’s Satchel is removed in exchange for this ability. This attaches rockets to several players, causing them to shoot out a barrage of rockets in indicated directions, inflicting a massive DoT to anyone hit as well as knocking them back.
Tick-Tock Cannisters - Gallywix ejects volatile cannisters around the arena which embed themselves into the floor. These detonate after 5 seconds, dealing light damage, split amongst all players soaking each one. All targets hit are afflicted with Mechengineer’s Cannisters, once again causing the same effect from Phase One and Two. If any of these fail to hit at least a single player, it instead affects the entire raid with their effect per unsoaked Cannister.
Gallywix Finale Burst - Gallywix finishes Suppression with pizzazz, firing a larger shock at a targeted location that deals lethal damage to all players, but is reduced by distance from the epicenter.
Ego Check - On Heroic this replaces Trick Shots, causing Gallywix instead to deal a 4-hit combo that deals moderate damage with each strike. This cannot be dodged or parried, and 90% of the unmitigated damage is dealt as a DoT called Checked Ego that lasts 22 seconds, and it can stack.

Strategy (Normal and Heroic)

It will be difficult to provide a working strategy without knowing exactly how the Giga Coils and Giga Controls work. From the wording of the dungeon journal, I assume each Giga Controls requires 2 Giga Bomb Detonations for it to break. The wording of the Giga Coils cast itself is also strange, as this would cause incredibly heavy raid damage in addition to all the other effects going on. For now, we’ll assume the following: The Giga Coils cast simply causes a set of Coils to radiate damage until interrupted, but Giga Blast will occur even if it is interrupted. Also, only one set of Giga Coils will activate at once, unless the mechanic is not done correctly and Gallywix is able to activate another set. This seems to be the case as the Overview section states he casts this as full energy. We will also assume that multiple Darkfuse Technicians spawn at a time, and so you need 2 players to fully destroy the Giga Controls. However, it may be possible that Gallywix casts this often enough that you will want to delay fully destroying a Giga Controls set until necessary, in order to keep the room from being eaten up by Sabotage Zones.

Also, we will assume that in the intermission, the abilities are used multiple times, as the Armageddon Class Plating shield amount seems incredibly under-tuned otherwise.

Lastly, we will assume that in Phase One the boss is stationary, as otherwise the description text for Phase Two makes little sense.

Phase One

The simplest phase, as there are no Giga Coils to deal with. You will want two teams to rotate through soaking the Scatterblast Cannisters. Evens first, Odds second is a tale as old as time, and will be the simplest way to deal with this.
You will need a large healing cooldown to heal off the healing absorb for each one of these Cannister soaks. The boss is stationary, but for the least movement required you can have each team stack with their tank and simply have the tanks taunt for each soak so no one has to worry about not being in the right spot.
As this soak may overlap with other abilities, it may also be helpful to set static soak points so that players can dodge/bait mechanics away, then return to those areas for their soak.
During the Big Bad Buncha Bombs, you will want the raid to spread a fair bit away from each other to give room for dodging. For Sapper’s Satchels, players should dodge to the sides of the arena for this, especially on Heroic, so that there is little chance of complicating movement for others. Anyone who doesn’t have the Satchels should dodge towards the middle of the arena.
On Heroic, tanks and players with immunities should soak the 1500 Pound DUD as soon as they can, but try to do so in a staggered fashion, so that the Time-Release Crackles don’t occur all at once.
Tanks can swap for Trick Shots around 5-7 stacks or just swap for each Scatterblast Cannister, whichever occurs first.
There will likely be an overlap where there will be a Scatterblast->Big Bad Buncha Bombs->Suppression->Venting Heat, causing an incredible amount of raid damage while requiring a great deal of movement. This is where raid CDs and personals should be used, as well as movement CDs.

Phase Two

From this point on, the fight becomes about managing the Giga Coils and preventing them from dealing their massive damage to the raid. However, you must make certain that you balance this while keeping the play space open for play. For now, we will assume that you must bring 2 Giga Bombs to the Controls for each Giga Coils, but keep in mind that you may be able to simply do 1 Giga Bomb at a time to keep the area as open as you can for as long as possible.
You should have players assigned to pick up Giga Bombs, although anyone who understands the mechanic can feel free to do it. Players carrying Giga Bombs should be careful not to be struck by the Giga Blast, as this will not only damage them, but destroy the bomb, possibly preventing the raid from stopping the Giga Coil cast, likely causing a wipe.
The tank not currently tanking should pick up the Darkfuse Wrenchmonger and any other adds and bring them to the boss for cleave. The tank who has Gallywix can assist in this as well as needed. The Wrench stacks should not be a great concern on these difficulties, and worst case you may simply need to pop something. These should be cleaved down.
Players should make certain to interrupt the Sharpshot Sentries, as their Shock Barrage cast can combo a player taking damage from other mechanics.
In general, during a Fused Cannisters, players should try to loosely stack together so that pairs of players can be found as soon as possible. Should this overlap during a Big Bad Buncha Bombs, it will be quite difficult to manage finding a partner with not stacking with a player with Sapper’s Satchels. It is good to keep in mind the previous rule, in which I recommend that players with Sapper’s Satchels should move towards the outside, and everyone else should move inside.

Intermission

As mentioned, we will assume that these abilities will be cast multiple times due to how undertuned Armageddon Class Plating is.
You will need some sort of movement speed for every Cratering, ideally.
Players should make certain to stay as grouped as possible for healing while breaking the shield, and on Heroic should also care for the Radiant Electricity beam.

Phase Three

This plays much the same as Phase Two, with some slight changes.
Players with the Overloaded Rockets should still follow the same pattern of dodging out of the raid towards the outside, and they should in general try to go to certain areas to make it so the Rockets all come from the same general area.
When the Suppression happens on Heroic, you should ideally have players bait the final blast as far from the raid as possible to minimize movement, but if it is not controllable in that fashion then you will simply need to make a large movement away from it.
Movement should be saved for the Tick-Tock Cannisters, so that players can get to a soak in time. Players should simply get to a Cannister and plant. Make it so that if another player sees you, they can simply focus on getting to a different Cannister.
The tank who receives the Bigger Badder Bomb Blast should move to a far off location to explode. As this actually does not deal that much damage, each tank can probably take 2 stacks each before needing to swap, although it is not known if the debuff will refresh each time. For now, alternate taking each one.
Ego Check is also not that much damage, and each tank can probably take multiple, depending on what they are comfortable with and what they have after soaking the bombs. For now, I would simply recommend swapping on each Ego Check.
Towards the end of the encounter, you will have much less space for dealing with all of the above, and there should be a much greater focus on player’s baiting properly. Tanks may also take the Bigger Badder Bomb Blast into the Sabotage Zones as space dictates.

Mythic Changes

It is assumed that there will be a mythic only phase as of this writing, and so we do not know the full breadth of all the changes on mythic. However, there are changes we can see in the dungeon journal for now.

The encounter begins in Phase Three, but also has the Darkfuse Wrenchmongers from Phase Two instead of just the Technicians.
The Cannisters have now become Combination Cannisters, doing a mix of Fused Cannisters and Tick-Tock Cannisters. This means you will need players to not only find partners for their soak, but also soak the embedded Cannisters in the ground, further complicating movement in between all other mechanics.
Darkfuse Technicians are now Giga Juiced Technicians, and have new abilities such as Ionization a debuff it places on random players that can be dispelled and Static Zap, an interruptible cast on a random player that deals damage and stuns them for 4 seconds.
Darkfuse Wrenchmongers Lumbering Rage now increases their damage by 500%, likely requiring the tank to kite them if this is not dispellable.
Giga Bombs now start as Discharged Giga Bombs, and thus do not detonate until they are Charged.
Any Giga Bombs, charged or not, trigger Auto-Locking Cuffs 0.5 seconds after pickup. This prevents players from using Fling Giga Bomb or from picking up additional Giga Bombs. If this aura removes for any reason OTHER than a Giga Bomb Detonation, it detonates, dealing lethal damage to the entire raid.
Control Meltdown, which interrupts Giga Coils, now only lasts 15 seconds, forcing players to protect the Giga Bombs for a much longer time. This is in addition to Giga Bombs now also taking 3.5 minutes to detonate, up from 30 seconds.
Giga Blast now leaves Giga Blast Residue behind for 21 seconds. This either charges a Discharged Giga Bomb if a player carrying one comes into contact with it, or stuns a player not holding one for 8 seconds and deals light damage to them. Either effect will consume this Residue.
Giga Bomb Detonation also deals damage to the control based on the health of the bomb, likely referring to the amount of charge it has. This means you may need several Giga Blast Residue to fully charge a bomb.
In the intermission, several players, and any players between them and the boss, will be tethered to Gallywix, dealing a light amount of damage to them that quickly increases to a large amount of damage based on how far they are from Gallywix.

Mythic Strategy

The entirety of Phase One of mythic now revolves around protecting and charging Giga Bombs. Any player that picks these up and dies with them for any reason will wipe the raid. You must also now charge each Bomb potentially multiple times before you can actually blow up the Giga Controls. You must also time sabotaging the Giga Controls so that Giga Coils will actually be cast while they are in a state of Meltdown.
Depending on how many Technicians spawn, it is possible you may need to only partially charge some Bombs simply to get them off of the arena before they can detonate, if you end up with an excess.
Adds become a much higher priority as each is in general much more dangerous.
It is not known when exactly the Intermission actually occurs at now. This also requires far more strict positioning, likely with all players now needing to spread out into their own designated locations for movement during this part of the fight.

Notes on Healing

Like Mug’Zee, there is far more healing required in this fight than all the others. There is constant rot throughout, with moments of high burst damage to the raid, spot healing, and triage. Almost all healing profiles will be seen throughout this encounter.

The raid will alternate being stacked and spread, and understanding these moments will be at the core of you properly healing this encounter. There will also be moments of high movement at which you will also need to do a large amount of healing. For each Big Bad Buncha Bombs, you should factor in this movement and be ready for it and have a plan for how best to do your healing during it.

Notes on Tanking

Boss positioning will be quite important throughout this encounter. For the moment, we will assume Gallywix will activate the nearest Giga Coil, giving you some control over where the fight occurs at. On Heroic, you also have a greater responsibility in the 1500 Pound DUDs, which require you to go out and clean them up while you are not tanking the boss. You need to do your best to keep the boss in a clean area for the melee, and make certain adds are picked up and brought to be cleaved down.

Defensive Usage

Defensives are best utilized during the Scatterblast Cannisters in Phase One, if you receive a Sapper’s Satchel in any phase, during the Big Bad Buncha Bombs or if a Giga Coils is allowed to persist for some time.

Movement Cooldowns

Movement Cooldowns will be very impactful in this encounter. You will want to use them, for the most part, during the Big Bad Buncha Bombs to aid in properly spreading out Sapper’s Satchels and Overloaded Rockets, or for the Fused/Tick-Tock Cannisters.

Damage Profile Best Suited for this Encounter

As of this moment, this does seem to be another encounter where Single Target with efficient cleave is best. As we do not know how many adds will spawn, it is difficult to estimate how much cleave and potentially AoE is necessary, but Single Target will still be king as of now.

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