I have managed to kill several monsters in dungeons where there are lots of walls and get money enough to buy some hit points. There's a dungeon -- I think it's #4 -- which is a straight shot to the hidden merchant. From there you can just keep firing arrows, getting gold, buying more arrows, etc.
But since you can't use the merchant when a monster is spawned, eventually you run out of arrows and bzzzzzt -- dead.
Flasks of oil are nearly as good as arrows for clerics and wizards. The KILL/KIHL spell is decent, if you can afford it -- 500 gp for a 1 out of 3 chance to kill a monster outright. Wizard Teleport is a great way of getting to the secret merchant. So a Wizard with a high strength and a lot of gold can do quite well.