i thought i would share this somewhat cheap but very easy and totally effective lich-killing technique. it has worked on every lich i have fought thus far (but i haven't tried the final kangaxx form yet)
supplies: one mage with the 6th level spell "power word silence" + a few fighters with quick weapons and perhaps an archer or two.
you can do this even without "cheating" (attacking while the lich is still blue) - it seems to work best if you have autopause set to pause on sighting enemy. or of course you can start while the lich is blue, whatever your preference.
the very first thing to do (while the game is paused) is cast power word silence. there is no save vs. this spell, so the lich will be immediately silenced for 7 rounds. once the lich is silenced, you can have your fighters approach, slice him to pieces, and 9 times out of ten you will kill the lich without it ever firing off a single spell. it may have a protection contingency pop up at some point but if you can manage to keep pumping it full of lead and arrows, it will die before it can time stop or gate or imprison. (p.s.: while using power word silence, do not attack with the holy avenger. its dispel magic effect counters the silence, thus negating your protection.)