The fight is truly trivial due to the way stealth currently works. (Warlocks Eldritch Blast and hex really helps as they have no force resistance)
1. Go in kill the 3 imps.
2. Go upstairs.
3. Kill 3 imps.
4. (optional but makes stuff easier) shove the devil 1 or 2 times to position him downstairs.
5. Go to the "gallery" further upstairs (1 elevation level above the thingy you need to press)
6. Proceed to stand in a "lightly obscured" area (symbolized by half sun cursor) from which you can range attack the devil (either a bow or Eldritch blast)
Repeat below until all 3 devils are dead:
7. Range atack.
8. Hide.
9. Go back to 7. Unless the combat ended.
You get xp only for devil kils (mindflayer gives 0)
Takes around 10 turns? Depends on luck and when the mindflayer dies.
