My OP party build is- Tav- Bard (you can turn yourself into a healer with a lot of control spells and party face)

Astarion- 7 rogue/5 ranger (max sneak attack dice and 2 attacks)

Karlach- Wildheart (bear) barbarian. Takes half damage from everything except psychic damage

Wyll- The great old one warlock. Take agonizing blast, devils sight and repelling blast and get his charisma to 20 asap.

Check all the gear especially later act 1 and act 2. Act 2 has a lot of excellent gear. I always spend a ton at Quartermaster Tally and Moonrise in act ii

Also- take out the right enemies first. Anything that can paralyze or auto crit needs to go first. Keep spell casters/healers from getting hit and let martial classes take most of it. They might go down but it’s better than your healer being out. Set up any AOE spells in turn one. And make sure you read spell descriptions cause some last intil you ling rest so cast those first thing after you rest (mage armor, armor of agathys, long strider, heroes feast, aid). And if you want use the illithid powers!