He likes to sheep and be annoying but his defining characteristic is a full-room Arcane Explosion that does around 5k damage. It's got a long casting time and is easily interruptable - but occasionally when he's about to die, he uses his other defining characteristic, Arcane Bubble. This makes him immune to everything, including spell interrupts. I'm told that it is dispellable, but not purgable. (Don't get me started on how stupid that is.)
To counter this, if he's got bubble up, and you can't get it dispelled, everyone should get on their toes, and as soon as he starts to cast his AOE, get out of LOS behind the nearest pillar. You should have time to get out of LOS before he nukes you.
Final boss in underbog (fel strider looking thing) has an ability to levitate you for quite some time but you can still do stuff at range. Sucks if you are a meleer, but if you're a caster, ranged etc, don't stop working.
Big archon looking things in Mana tombs have a nasty fear effect, but can be stun locked, doing so helps
2nd boss in Slave Pens is the king crab. He does a frost based AoE bolt spell as well as a huge dot that doesn't go away until the toon is healed to full. Annnd also does a mold thing, but I dunno what that does. Just stay beyond 30 yds and you'll be fine as the AoE is limited to 30 and the DoT is melee.