Ruins of Mazzatun

The Xal-Nur fight definitely requires a healer and 1 person to collect his poison (spice) and run it to a geyser to purify it. Xal-Nur is invincible and charges you while hold the spice. The longer you carry the spice, the slower you get.
The Tree-Minder Ka-Nesh fight requires someone to mark/destroy statues and totems

White-Gold Tower

The Adjudicator boss can throw you in a cage where you will burn while you're trying to unlock the door to get out.

The Planar Inhibitor boss requires the tank to grab a pinion in the beginning of the fight and have a team mate grab it when the flames under the boss turn from orange to blue. The blue flame will wipe whoever it touches and after kills a player, it fixates on to the next player etc etc. When she is fixated on you when she's blue, you become increasingly slower as time goes on and she eventually gets you with her flames. Plus you have portals you have to destroy and if you don't, you'll have a room full adds wiping everyone

Imperial City Prison

Ibomez the Flesh Sculptor will turn invincible and summons zombies who walk towards the middle of the map to an area filled with poison. You have to throw these poison pod/grenades at the incoming zombies. The zombies come from every direction...too many to kill by yourself. If they reach the poison pond, they turn to flesh atronachs .

To get to the last boss, you need to throw switches and it requires two people to accomplish this.