[Radek]

If you have to deal with liches on rather low levels (for example, in chapter 3):
1. Send one of your guys to trigger the lich. Give him Boots of Speed or cast Improved Haste on him. When the lich appears, let the guy run back like hell. The Time Stop cast by the lich is wasted.
2. Now, send summoned monsters to the lich one by one. Most of powerful spells cast by the lich are wasted.
3. If your level is not too low, send Mordekainen Swords or Skeletal Warriors to visit the lich. The lich gets much deader than it has ever been. Otherwise, you can attack him directly by your weapons (they must be +3 at least) or spells. The lich is not so dangerous now. At any rate, cast Remove Fear and Chaotic Commands before approaching the lich.