When you’re looking at party composition don’t neglect constant buffs. A paladin or a chanter makes your party vastly more effective and your priest should be buffing as much or more than healing. When your first cast from a priest is a heal, you’re losing the battle. If you have Xoti (who I probably won’t take again), make sure to upgrade her lantern with the AoE buff after you do her quest. I would recommend either a paladin or a chanter.
I would recommend the following composition:
1 physical dps (ranger, rogue excel here.)
1 - 2 off-tanks (fighter, chanter, paladin)
1 healer (priest or paladin, druids are less effective because they can’t provide critical intervention)
1 - 2 controller (mage, cipher, druid, specialized priest).
0 - 1 AoE damage dealer, normally the same as your controller.
In reality if you want a truly effective most characters will be doing multiple roles.
My current party is a thresher on hard; My main is a Devoted / Soul Blade - off tank, melee dps, occasional controller. On hard fights he’s right in the center; on easier ones he’s my mage killer.
Eder is next as a full fighter, and he’s built for both damage and resilience. I used to use him as a main tank, but he’s pretty good at locking mages down. (Sabre and shield).
Next I have Xoti; her responsibilities are buffing, healing, AoE dps and melee in that order.
Aloth is full-on wizard controller / AoE dps.
Last but not least I have Pallegina. Normally I keep her in the back on her excellent pistol, but she’ll replace Xoti on the frontlines with a difficult fight. I built her for few active abilities. (Pistol / Great Sword).
Xoti and Pallegina can be replaced, in that order. Takehu (pure druid) is my most often sub, but he is a little underwhelming as a controller. (Getting summons to take the heat is hard). Next time I play through I’ll probably be a monk / paladin.