Have you any flying foes, or some that can teleport on short distances, like in within a room.
To me, the monsters that were the most challenging (annoying but so satisfing to kill <img src="/ubbthreads/images/graemlins/delight.gif" alt="" />) were always those that regenerate themselves. Like those trolls in Icewind Dale that you can't kill except with a fire/acid enchanted weapon or when they're quite dead they get inert and wake up full health again after a little while...
