Use scrolls of familiar summoning in DH2 until you get GEE.
Order it to slay all liches, collect the artifacts, move on.
You can also lure them into a corridor and use bouncing magic missiles or fire bolts, but they will heal often so it might not be such an ideal case.
This method also limits the amount of monsters they can summon to a fraction of what you normally get.
Preferably, use both methods - GEE in a tunnel between you and liches.
Once they see you, whey will use their great tracking ability and will even find you on the other side of the level, if that's where your favorite corridor is.
@Soirana: fire immune monsters isn't such a great idea anymore because of how frequently monsters swap places.
It's unreliable and the liches will quickly close to melee range and start draining.
"Hell is empty and all the devils are here."