he is Immune to all Mage spells below 6th level - Pierce Shield is the only thing that works on lowering his magic resistances. Nothing else will work.
He regenerates 3hp/second
Requires a +3 or better weapon to hit
He is Immune to Critical Strikes, deathblows, power attacks, all your "special abilities".
He can summon demons and outer planes demons/creatures as many times as he likes.
He attacks with level drain, he drains 6 levels if you fail your save.
He also attacks you with diseases on every successful strike - its the same as the contagion spell mixed with Delorious Decay its 2hp/second for 1 turn.
He is 95% magic resistant
He has 290 HP
AC -6
THac0 -2
Now this is just the Origianl guy - you mentioned he was "New and Improved" ? LoL why??
I'll tell you why, at least in my perspective. 3 hp per second wasn't really all that fast; Abi-Dazlim's Horrid Wilting takes down those HP's *fast*. Most characters, just by playing through Watcher's Keep, already have +3 weapons (more likely +5). Greater Whirlwind still works, and I rarely, if ever, need to use Critical Stikes against him, with a AC of 'only' -6 (I usually have a thac0 of -10 or better by that point).
Summoning demons isn't that much of a deal; just concentrate on the big dude. Mordy Swords can keep them off of your back is you're really desperate. Level drain is fought off by the Amulet of Power. Diseases, likewise, either by the Ring(s) of Gaxx, or by the Periapt Proof against poison. Besides, he has to hit you in melee for that to happen, and that's a rare chance against me; I usually just keep slinging Skull Traps and Abi-Dazlim's Horrid Wiltings at him.
95% MR? Not a Problem, expecially considering that most Mages, by that point, are level 20 +, and make sure to put in a triple Pierce Shield Chain Contingency before facing him. That leaves him with 5 MR. 290 HP goes really quickly. -6 AC and a thac0 of -2, compared to a character at that point, is almost pitiful. My Kensage has -5, just with the Robe of Vecna, the Ring of Protection +2, and the Ring of Gaxx. He also has a thac0 of -6.
I got a lot of practise against enemies with immunities to spells lower than level 6 a lot, especially because I was making a point of killing all of the Liches. Still, Skull Traps will work. And weimer even said that spells 3 and up will work, just not those of levels 1 and 2.