[Gammit]

I've found the best way to kill liches is to first set as many traps as possible in the area where the lich will spawn, and have monsters already summoned nearby. After the traps go off, I have the monsters rush in with my fighters close behind. My fighters can still hit the liches through one layer of monsters, and the lich almost always focuses his attacks on the monsters. Meanwhile, my mages bring down his protection spells. Ring of the ram can help out too.