Source: Dulfy (and thanks to all the Oops'lings who helped create the class-guides and changes to the boss-guide).
KEEP CONSTRUCT GUIDE
Enrage Timer: 10 Minutes
Class Composition: Keep Construct aggros on the player with the highest toughness. Good crowd-control skills are needed.
General Strategy: Keep Mesmers Projections apart and kill them near the Keep Construct to remove his Xera’s Embrace stacks and then break his breakbar to force him into the Construct Core phase. Push the Construct Core into Ley Rifts to gain stacks of debuff on the boss that increase the damage taken during the subsequent burn phase. Keep an eye out for AoEs, Crimson/Radiant Phantasms, and beware of the Magic Blast mechanic at 66% and 33%.
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Keep Construct Buffs
Keep Construct has 2 buffs you need to be aware of.
Xera’s Embrace ( ) Maximum of 2 stacks, this can be removed by killing the Mesmer Projections described below. Removing 2 stacks of this buff causes the Keep Construct to gain a breakbar and enter a special mechanic phase.
Gaining Power ( ) Each stack of this buff increase Keep Construct’s direct damage by 10% and condition damage by 3%. Keep Construct gains this buff passively every minute and it cannot be removed (you will see the message The Construct’s strength increases when this happens).
Starting at 9:40 on the timer, two mesmer projections will spawn from on statues on opposite sides of the room and become fixated on a random player similar to the fixation mechanic from Slothasor. Each of the mesmer projections will have a white circle underneath them, and if the circle of one of projection touches the other projection they will gravitate toward each other and merge to form an Insidious Projection, that has a lot more health and will convert player boons to conditions.
You can see where the next Mesmer Projection will spawn next, since the statue they will spawn from glow in a bright light. This give you a good idea of where to move the boss so you can AoE down the spawning projection. This is especially important when you have two projections spawning at the same time like at 9:40 or once the boss finishes the 66% and 33% Magic Blast phase.
Much like fixation on Slothasor, the player being fixated will have a purple icon above their head and see a message displayed across their screen – You have been fixated
if the fixated person get downed, the Projections will switch their fixation to someone else in the raid. This can cause projection merges if the newly fixated person does not pay attention.
Getting one Insidious Projection to form is not huge an issue, but if you keep getting them then you have a high chance to fail, since they have much more HP, hit hard, and do not disappear.
Players should pay extra attention for this fixation mechanic and move away from each other to prevent the projections from merging. Once they start merging to form the Insidious Projection, you cannot stop it.
The first 2 mesmer projections will spawn around the same time, but the ones after that will be staggered by about 15 seconds until the boss reaches 66% or 33%. The first 2 projections that spawn after 66% or 33% phase always come in pairs and then become staggered again.
The projections should be dragged to the boss and killed with AoEs while keeping them away from the other projections. Each projection killed near the boss will remove a stack of Xera’s Embrace buff. Once both stacks of Xera’s Embrace buffs are removed, the Keep Construct will gain a break bar. Breaking the breakbar will force it to enter the Construct Core phase.
Construct Core Phase
Construct Core Phase is only encountered if you remove Keep Construct’s Xera’s Embrace buff and then break his breakbar. During this phase the first thing you will see is 3 green circles split across the room. Each green circle will have a Xera’s Phantasm standing on it trying to protect the core. All you need to do is to have 2 player or more standing on each green circle, and the Phantasm will be destroyed once the green circle times out. Failing to do so will result in a raid wipe.
What to do (NOT)
Once the Phantasms are destroyed, Construct Core will be visible and players can move it around by using their attacks. 5 Unstable Ley Rift will also spawn around the room with an outer red ring enveloping the room. The goal here is moving the Construct Core into the "Unstable Ley Rifts" by hitting it. Each Unstable Ley Rift touched and destroyed by the Construct Core will cause the Construct to gain a debuff that increase your damage by 75% per stack in a later burn phase.
Moving the Construct Core to the Unstable Ley Rifts is not an easy task as Retriever Projections will also spawn and try to retrieve the Construct Core. If the Construct Core is retrieved by the Retriever Projections or get knocked into the red outer ring, this phase will end immediately. Otherwise this phase will end only when all 5 Unstable Ley Rifts are destroyed by the Construct Core.
1 player should be assigned to pushing the Construct Core around with their autoattacks knocking them into Unstable Ley Rifts around the room. You can have mesmers to help pull the core into the last rigt (or other classes with pulls), Just be careful that you don't pull it out of the circle.
The Retriever Projections that are following the core-pusher, will despawn once the Core Construct phase ends so there is no reason to kill them.
Burn Phase follows immediately after the Construct Core Phase. Depending on how many stacks of debuffs the Construct get from the Construct Core Phase, you could be doing as much as 5×75% more damage to the Construct so all your big DPS cooldowns should be saved for this.
The Construct have a ton of HP that absolutely requires these burn phases so you want to get ideally 5 stacks of the debuff from the Core Construct phase or a minimal of 3 stacks on average to prevent an enrage.
Radiant and Crimson Phantasms
Depending on your DPS, you may not see much of this mechanic but it is worth mentioning as doing it incorrectly can result in a raid wipe. Radiant and Crimson Phantasms will spawn from time to time and they all need to be killed under a certain time limit. If you don’t kill them within the time limit, they will explode and cause massive raid wide damage. Radiant Phantasms will have a white attunement icon above their head and Crimson ones will have a red icon. Players in the raid will also get a white or red attunement icon above their head and they can only damage the Phantasms with the same color as their attunement.
You need to either kill these phantasms quick before they blow up the raid, or you push the fight quickly into the Construct Core Phase which will cause these Phantasms to despawn. This is simply a raid awareness mechanic as the phantasms have very little health and die quickly.
Keep Construct has four kinds of AoEs that are present in the fight.
The first is a large orange circle that indicates the area the Construct will smash/jump to. This is very easy to avoid but deals quite a bit of damage. It is also a bit infrequent.
The next AoE is a rotating AoE attack, much like the flame wall on Sabetha (although much less deadlier and won’t instant kill you). This attack is only a single triangle at high health % but evolve to a more interesting double and triple triangle pattern at low health % that can down a few players if they are not paying attention. Circle strafe attack around the boss in melee range is usually the best option to allow your healers to top you up. Be careful that you are not inside the little circular area in the middle as it also deals damage and can down you quickly.
The third type of AoE is a raid wide exploding bomb much like the one on Matthias. The difference here is that the circles are much larger but you have plenty of time to get away from other players and not blow up on each other. Each circle you are standing in cause you to lose 20% of your HP.
The last type of AoE is typically seen under 25%. Construct will jump up and then rain down small circular AoEs on the ground that are easy to avoid.
This is a special mechanic only seen at the 66 and 33% phase. Keep Construct will raise his hands up on the air and you will see a message on the screen – The Keep Construct is attracting energy to unleash Magic Blast. When this happens every raid member will get attuned to either white or red and then red/white orbs will start moving from outer edges of the room towards the Keep Construct.
The Keep Construct will be immune to damage and everyone’s job during this is to body-block the correctly colored orbs from reaching the Keep Construct (red attuned members block red orbs etc.).
Crimson attuned - red
Radiant attuned - white
You will take a small amount of damage for each orb you block and absorb, but any orb that get through to the Keep Construct will grant him a stacking buff that will increase his Magic Blast damage at end of this phase (caps at 35 stacks), but you want to keep it under 10 if possible.
As long the Keep Construct doesn’t get very high stacks the damage from Magic Blast is very minimal.
Some Lulleh-art to clear up things!
Oopsy Video from KC with Achivement