SK or the console is your friend I guess.
I just tried out my suggestion with a 12th level Sorcerer (10th level when exiting Irenicus dungeon). I went round the perimiter as I suggested, using invisibilty and fireballs and one Cloudkill to eliminate his support. Took out the bandits whilst I was at it. I then cast (in order) Melf's Minute Meteors - stoneskin - strength - Haste - Draw upon Holy Might and finally Tenser's Tranformation. MMM's hurt him, I then finished him off with my +1 staff. No contest, he didn't do any damage at all. The lowest roll I needed to hit him was 7 (+9 for a total of 16), a 5 missed, never rolled a 6.
The fact that MMM actually hurt him underlines the fact that it is spells he is immune to (up to level 7) not elemental or magical damage.