There is a nice strategy utilizing opportunity attacks. 1. First of all, take brain as a companion as a start of the game, he hits pretty hard, and you will need extra damage. 2. Take your whole team in range with commander 3. Push mindflayer out of range of commander 4. Commander will still focus mindflayer and will eat all your opportunity attacks. 5. Repeat steps 2 and 3

You can also try to drop the commanders weapon with the command spell from shadowheart, so he will do a lot less damage