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I've finally made it to the end of HotU, but i'm having some serious trouble trying to damage the big Mephisto. He's definitly got some killer damage resistance and immunities going on. My character is an Epic Sorc (22), Red Dragon Disciple (4) and Pale Master (1). I have a reasonably well varied spell range but so far nothing except for Negative Energy damage (Finger of Death) has been able to hurt him. He's even immune to my +10 weapons! Can anyone give me some hints on, a) spells and effects that can hurt him or, b) how to lower his immunities.

While we're on the subject, i've noticed that quite a few enemies (including the big M) have a cheesy negative energy type spell/ability that they seem to be able to use, at will, several times per round. What is it?

It has been a while but I think acid damages him. So use acid damage spells and make sure your +10 weapons also have acid damage on them.

Knightscape is right about the acid. It is one of the only thing that damages him. Also, if you had enough money to buy his true name from the knower of names, you can just banish him from the material plane without a fight.

No! Not acid! I've only got one acid spell in my repertoire and it's a smeggin' cantrip! Think i'll have to dig up an archive save back in Hell and do some shopping.

As for the true name thing, The Knower of Names wouldn't tell me Mephisto's. Do i need a minimum INT or WIS attribute to unlock her dialogue? Not that i'd take that path though. ;)

Getting the knower of names to tell you Mephistos' true name isn't linked to wisdom or intellitence. I think you have to buy her true name, then you can force her to reveal Mephistos' true name.

I had so much extra gold by the end of HOTU both times I played through (first with a monk, then with a fighter) that I just went ahead and bought everyone's true name that I could, just to see what the different possible outcomes would be. With his true name, you can banish Mephisto from the prime material plane forever, make him your slave and terrorize the world yourself and maybe another outcome, but I forget. I ended up fighting him too, just because with a monk and a fighter, it seemed like the way to go. If you are lawful good, then the banishing might be the way to go, since even if killed, he just goes back to the outer planes of Hell, or where ever, and potentially he could return to attack the world again. The banishment is forever, so that removes one him as a direct threat. You still get the XP points for that too.