Source: Thanks to RandomDragon and the other Oops'lings who helped create this guide
Make a marker very close the the boss, where all should stay stacked (this way you also avoid his shard attacks). Very close on the other side of the boss, those with agony circles should stay close to each other, but still inside the boss.
This boss will mostly stay at mid and won’t move, also the CC bar never goes up so you don’t need CC for this boss. Cairn is not a DPS-race, so you can easily bring 3 healer classes.
Important thing for all the battle:
- You can use your SPECIAL ACTION KEY any time you want, it has a little cooldown (5 secs). It’s a cool teleport ability that you can use to move around with.
Enrage timer: 8:00.
Enrage mechanic: Debuff you recieve no longer ticks down when you go into grene circles.
Challange Mode: Your special action key will activate by itself, so you have to use it every 10 sec (you get a blue buff, counting down from 10 sec after each use).
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DEBUFF. You get a debuff there over time will slow you down. When it reach 99 stacks you get petrified and can't move. All your skill will vanish and the only skill you can use are the special action key. You can remove stacks by being in the grene circles when they appear.
Red teleporting circles:
Just under Cairn, you will see like a symbol craved on the ground. Inside this circle he will spawn constantly 3 red little circles, that will port anyone stepping into them (just like blue AoEs at VG). Be careful because it port you to the very edge of the platform. You can use the special action to get back to the boss instantly.
Every second melee attacks he does, he will cast some shards that go from him to the very edge of the platform. He spawns them in an angle of 120º approx., and it can be kited just like Sabetha, as they are directed to the most far away player. The shards do some damage and push you a little back. Be careful because he can throw you off the platform with this attack. Staying at melee seems safe.
You can reflect these shards, but if you are close enough to him, they will not hit you. All should stay stacked at the boss, all without agony together and the rest on the other side to the boss (still in close range).
From time to time he will cast an AOE that will get you up and down like well of gravity. For evading this attack there will be some green AoEs around the area. It usually goes through a cycle of this:
Normal green: Just stand in there and you are protected for the AoE.
Numbered green: For these you will see that greens are of different sizes, and have some numbers of dots on them (1, 2 or 4). Those means that there must be AT LEAST that number of people on them for it to protect. That means, if there are only 3 people in a 4-dot green, they will be affected by the AoE.
Jumping: For these Cairn will “tele-jump” around the platform, spawning green AoEs where he is landing. BE CAREFUL at this one, because he can sometime jump over another green AoE and push everyone off the platform if they are in that green.
People with the agony debuff should stand in their own little circles and everyone else can stand in the big one.
Procs every 20 secs (starting around 7:37) the furthest person away from the boss (who’s not yet with the shared agony) will get a damage AOE around him, similar to Matthias corruption. YOU CAN’T CLEAN THIS. It will disappear after some time. Be careful to not kill your friends.
Be very careful at this one because it’s a simple mechanic but also very easy to fail if you are not careful. He will land his sword to the ground and whirl counterclockwise. You just need to dodge it once, and if you don’t, you will be pushed back and probably fell off the platform.
When he reach 20% of his HP he will start doing this attack from time to time. He will spawn AoEs, starting near him like a wave and moves out to the edge of the platform. They will push you back, meaning you can die for it if you get off the platform.