On the Nataloid:
Surge forward killing the minions and dog pile the devil.
When his health is low, send one character forward to trigger the next group. As long as their are enemies the mind flayer will not attack you.
Finish Zariel and the mind flayer will move to attack the imps. He will do so until the last two devils arrive.
Use Shadow Heart's Guiding Bolt spell (x2) on one of the devils and focus fire on that devil first, and then the next.
If the mind Flayer survives he will be quite weak (avoid his mind blast by splitting up).
Hope this helps.
