You can tempt a lot of the wandering monsters a few at a time out into the anteroom and clobber them piecemeal. The real problem is the Eyes in the corners of the room, because they do a lot of damage, and they can see you and hit you from a lot further away than you can see and hit them. If you have a priest who can cast Poison Cloud, or some wands of Poison Cloud, then you could attack the Eyes by getting in at close range and entering turn-based mode. Try and arrange it so that the cloud does not disperse after the first hit, but hangs around the eye and does repeated damage - e.g. try and back them up against a wall.
BTW, the room next to the anteroom contains a black chest, which refills every time you leave and re-enter the lich lab. If your Disarm skill is high enough to open the chest safely, it can be very rewarding... 
