I think doing my first playthrough with only mage type companions and my latest focusing on melee/physical really helped me not worry so much.
The crux is really just mastering positioning, knowing your enemy and using the environment against the AI.
For example, you've got 3 melee characters and a rogue. Before you is a door. You know the room is full of ranged DPS characters. What do you do? --> shoot the first in the back, close that door and force them to come to you!
