In almost my first game of BG2 I used an undead hunter. Very effective, so much so I didn't use one again -- vampires become just too easy to kill. Not that they present much problem now.
The weapon I believe you want is +2 Azuredge, available from Bernard. A throwing ax that kills all undeas -2 or be destroyed. This is also ugradeable with illithium from Cromwell. The upgrade turns it into a +4 weapon, undead must save at -4, and it also protects you from level drain.
When thrown it gives a huge advantage against liches. For instance the "hole in the wall" lich.
Set the game to pause when enemy sighted. Enter lair, game pauses, throw axe. Run!
Repeat until Lich is dead.
A similar tactic can be used elsewhere. Halt just outside the liche's lair, cast Farsight. Now target Lich. Your character will now run into the room and hurl is ax the instant he sees the lich, usually before any protection goes up. You have to be a bit careful with Gaax however, you must allow him to have his little speech or you'll never kill him.
It's also the best weapon to use against he infamous Gaax, being +4 it will kill him, and used as a throwing weapon you don't fry yourself on his fireshield. The unmodded Bodhi is easy to kill; I've killed her by accident in Spellhold!