I hit the 2 million XP milestone inside the Temple Ruins and now there's a Lich accompanied by Greater Mummies and Skeletons just south of the Lava Room. The first time I walked in - unaware of the danger - the Lich made short work of me.
The strategies I've found on the Internet thus far consist of: 3x Spell Immunities, 1x Globe of Invulnerability, 1x Scroll of Protection from Undead, 1x Death Ward, 1-3x Pierce Magic, 1x Pierce Shield, 1x Ruby Ray of Reversal.
Of these, I only have the Scroll of Protection from Undead, the Spell Immunities, the Globe of Invul., and Pierce Magic. I only have one 7th level spell, and I'm saving it for Mordenkainen's Sword (as it's a +4 weapon and has great immunities itself). My mage is level 15.
It's a tough fight, but manageable.
Here's what I did: as the Scroll grants me immunity to his colleagues, I've decided to ignore Protection from Evil, Shield etc and focus my first level spells on Magic Missiles (for after the Pierce Magic spell) hoping to do some damage. Second level spells, I took all Melf's Acid Arrows I could for the same reason. Third level, Skull Traps. Fourth level, Spider Spawns (I'll explain in a bit). Fifth level, I only have two slots free (the others are occupied by the 3x Spell Immunities), so I chose to take 2x Animate Dead. The remaining slots of sixth and seventh level are Globe of Invulnerability, Pierce Magic and Mordenkainen's Sword.
My strategy thus far consists of baiting the Lich with summons, hope that it will fire off most of its spells (and keep on baiting for as long as necessary), MAYBE wait 20-ish turns for his protections to run off, and then finally engage (with all the Spell Immunities up) with a Pierce Magic as an opening.
It worked, sorta. The baiting performed as expected. I charged in, fired off my Skull Traps (taking out the Greater Mummies, but not the Skeletons). The Magic Missiles, Melf's etc did NOTHING to hurt the Lich, but he was all out of spells anyway (his Spell Trap had gone off, too). But then, I was out of spells, too!
What happened next is kind of hilarious: the Skeletons and the Lich ignored me - thanks to the Scroll of Protection from Undead -, and I just stood there thinking "what now?". Hehehe. My mage is carrying a +3 quarterstaff, and there was like 2-3 Potions of Strength and 1 Scroll of Improved Haste in my backpack (plus, at level 15 & properly geared, his thac0 is not half bad, sitting at a comfortable 11), so I POUNDED the Lich and Skeleton Warriors to death!!! And they just stood there. Hahaha.
Anyway, next time I'll memorize Improved Haste, Strength and whatever other melee enhancements I can think of, and just hit them in the head 'till it's done.
(The baiting worked, tho).