Prepare.
Make sure your fort saves are up to snuff--most necromancy spells with saves have fort saves. Fort saves are also what you need when facing undead.
Make sure you can deal with incorporeality. Have ghost touch weapons and armor or at least buy wands of Mage Armor and Magic Missile. Favor deflection bonusses to AC instead of natural armor as well.
Buy a few scrolls of Undeath to Death (Tome and Blood, 6th level wizard/cleric spell). It's expensive, but if you need to take out a horde of undead, that's the spell to do it.
And research. This necromancer has obviously been around for a while if you think he might be a lich. You may be able to glean some of his tactics and vulnerabilities from the stories. If he had any apprentices, find them and find out what he taught them. Pay special attention to any missing schools. After all, if he's a specialist wizard he'll have a prohibited school and he couldn't teach that to his apprentices. Once you have an idea of his favorite tactics, find yourselves some rings of counterspells or some other way to negate them.
You will have to be very well prepared. Lots of money and Undeath to Death can substitute for turning. (Power attack, cleave, and (especially) great cleave work for zombie and skeleton hordes too). However, there is no substitute (short of limited wish) for Restoration (or even lesser restoration) so your focus will have to be avoiding being hit by undead which deal ability damage, drain abilities (these usually have a save, so it's actually easier to avoid than ability damage--you're just screwed if you blow the save), and negative levels. I would consider ensuring that you are adequately protected (mage armor all around if you don't have ghost touch armor, etc.) and then focussing on all-out no holds barred offense against such foes. If you gain initiative and kill them before they act, they can't hurt you. . . .