The first boss is Bone Dragon. 1 party can handle him while the other 2 handle the adds. Upon the dragon's death, the skeleton adds will resurrect and run to the boss' corpse and explode dealing unavoidable 1500 damage. The idea is to tank them and kill them as far away from each other as possible. Also the adds if they are close when resurrected, they'll chain together and run faster so... kill them apart too. When they rise up, kill them before they reach their destination. The boss will resurrect 3 or 4 times I'm not really sure, but the adds will change and it'll gain new set of abilities. Nothing threatening.
Second boss is the split part. Each alliance takes its own path. There is a colored pad on the floor on each path that "lock/unlocks" the other party's boss. So the idea here is to split your party into 2 teams of 4, one team stays on the pad so the other party in the alliance can damage its boss while also dealing with the adds, while the other team damage the boss. If you fuck up, an Iron Giant pops and 9999'd everyone on all 3 paths.
The real second boss is Thanatos. He is immune to all attacks and only damaged when you gain a buff from the 3 magic pots in the area. Adds will pop and attack the magic pots so whoever doesn't have the buff that allows you to damage the boss, gets on add duty. That's about it.
Last boss I fought, but not the dungeon's final boss, were 3 Batraal type monsters positioned in a triangular formation, connected to a bomb. If you move them, the lines get deformed and bomb gets bigger until it explodes. Bomb adds also pop and head to the main bomb and does something to it and it gets bigger. At one point it exploded dealing 20k damage to all. The theory the team had was that all 3 must die at the same time so the bomb won't trigger as a final move. But we didn't manage to try it.