For most characters, I find you can just club him to death with the TotRR after throwing a potion of cure corruption at him. As long as you have a PV of maybe 40 or a DV of 80, he won't be able to hurt you. Slaying ammo is always good, although I find I rarely have enough to make a sizable dent in him unless I'm playing a very missile-friendly class.
I've never tried necromancy in the Chaos plane, so I can't say whether or not it would work.
For a chaotic ending, corruption control is more of an issue. You need to kill him reasonably fast if you don't have a ton of corruption removal stuff ready.
Any time a player finds Executor and fails to use it, the RNG kills a cute dog.
Hoping to win with every class, doomed. Archer, Barbarian, Bard, Beastfighter, Druid, Elementalist, Farmer, Monk, and ULE Priest down.