Are you moving your party out of Negation Blast? He makes one big shiny orb that will blow up the entire platforms nearby and mark allies affected within its aura. You are probably taking too long in the final fight as well if a Nautiloid is dropping reinforcements, the Emperor and Dragon should be dead by the first 2 turns and the other mindflayers should die by turn 3. while your Illithid is channeling the Netherstones on the portal.
That's simply not true, not even on Tactician. I was in that fight for ages just for the lulz, been camping spawns from the nautiloid for many rounds with all my allies summoned. I've been there for over 10 rounds, it has nothing to do with what happens inside the portal.
Once inside my Avatar just wrecked the brain, all I needed was for Gale to Misty Step, get into the portal and cast a level 5 Magic Missile with the +dmg gloves.
