Used this general guide as base.
DISCLAIMER: There are many ways to manage a fight. Just because I do stuff this way doesn't mean something else is necessarily wrong, as long as the boss gets moved, things get purged and impaired.
Haugbui Jarl
Run close enough on the right-hand side so that the boss is activated, let him come to you. Most likely, the first cast will be Charged Hack, which can be impaired, but preferably hold off on it until he’s on dry land, as he typically casts Deep Calling right after, and the less you have to go into the water for that, the better. Alternatively, you can also engage him on the island he’s standing on, but that’s going to mean water electricity damage for you and melee dps (if any in the group). When you see the boss start casting Deep Calling, run a circle around the boss to break line of sight and cancel the attack. If the jarl casts Deep Calling as the first attack of the fight, you still need to LoS it, just use Immutable and try to not be in the water any longer than absolutely necessary. Impair Mjolnir’s Echo, as this is an aggro reset.
Blarbane Sorceress
Ask ranged DPS to shoot the pods to clear the area before the fight. Purge Tide Wall. A couple adds show up from the exit side of the fight (behind where the boss first stood, coming up from the left hand side) and should be rounded up. Deathsquall cannot be interrupted, just wait for the cast to complete and move out of it. Technically you can also power through it if you’re feeling tough enough, but melee dps can’t, so don’t be that guy.
The Verangian
Dodge out of Deep Blast. Impair Charged Hack or on lower elites, have a shield up and eat it. Impair Mjolnir’s Echo which is an aggro reset. When he dives he’ll summon a handful of adds, round them up. When he flees, be sure to round up the two sets of 3 adds that emerge, before they eat your group mates. Once on the crates, same procedure – impair Charged Hack and Mjolnir’s Echo. He often does Charged Hack straight away, so if you’re using Evulsion to taunt, hold off on it so it can double as an impair.
Haugbui Mother
Move the boss towards the exit of the arena, so that your basic AoE gathers up the adds coming up from behind you before they have the chance to spread out. Move out of Corpulent Slam. Once Eversion is cast, move out of it and, if necessary, gather up the 3 adds that spawn, but they usually die in a second.
Primordial Dweller
Stand in the right-hand corner. Tide Wall cannot be impaired, but it can be purged, so if you’re using Evulsion to taunt, save it for Tide Wall. Bonus points for when the boss casts Synapse Spasm right after TW and you can get two for the price of one. Synapse Spasm should ideally be impaired, but depending on your mightiness and the E level you’re doing, you can eat it if you need to and have a defensive up. Psychic Death Zone cannot be impaired. He doesn’t always bring the shield up, though, so be ready to move back in. When the shield comes up, get to your DPS as quick as you can and round up the adds, using Immutable helps with aggro a little bit. The DPS should be instructed to not touch the adds before you've grabbed them, unless they absolutely have to and/or they’re ridiculously overgeared and can fend for themselves. After the adds are dead, face the boss again and get ready to run in on the right-hand side as soon as the shield comes down, making sure you have a defensive up as his first attack after coming up hits pretty hard, or be prepeared to impair Synapse Spasm. Do not try to pick up the two bombs that come out, they will just hinder and kill you, so let ranged DPS take care of them. Same goes for any adds emerging when the boss is active - if they come up from behind you and your AoE picks them up, great, but mostly the DPS' are on their own on this one, as the Dweller's attacks can chain off you to the DPS and kill them, so do not attempt to approach the left hand side. Rinse and repeat until the Dweller is dead.
The Ur-Draug
Get behind the boss and tank in the direction he has his back to at the start, away from the centre. Huge incoming spike damage. He also cleaves, so if you’re worried about losing aggro to one particular dps, you could consider bringing a taunt or two, as anyone pulling aggro likely results in the death of all DPS. Move out of Slam. Cosmic Gaze can and should be impaired – do it yourself or ask a DPS to; it has both a knock back and a fair bit of damage. Eventually he will cast Rending the Veil. Hide and use Immutable to regen health if you need to, you have time. If you see him looking at your rock funny, run out and catch Cosmic Sundering on yourself, be sure to have a defensive up at the time of the hit. Rending the Veil also wipes aggro, so if the DPS don’t already know, be sure to instruct them to not touch the boss until you have. You don't have to face him in the exact same direction he was before, but if you can, it's better to keep close to the original rock you all were hiding behind. But overall, just get to the boss and turn him away from the DPS' before he kills anyone. During the tidal wave and after you’ve been cleansed, move to the right hand side. He will most likely immediately cast Cosmic Gaze, which should be impaired once again. Don’t bother putting up and wasting a cooldown (unless you miss the impair of CG and need to keep yourself alive), as he will cast Slam immediately after. Proceed to kill boss.