Well I myself managed to overcome the Lich when I only had Drizzt and Wulfgar (after getting slaughtered a couple of times ofcourse...)
A good thing is to buff up with some haste either from potions of speed or a haste spell then also protection against electricity from either a potion or a scroll, healing potions ofcourse. +5 weapons is needed for dealing any serious damage to the Lich as I remember?
Drizzt has a +5 scimitar (Twinkle) and Wulfgar's hammer is a +5.
Surround the location with the whole party before the dialogue, then charge him directly after the dialogue ends (or if you wanna have a cheaty start then pause right before the dialogue starts and click attack so you can get some hits on him before the dialogue aswell
There are other ways ofcourse but this one worked for me (mayhaps that I was lucky.) But hey a Lich is a Lich