The monsters on Dungeon Levels 5–6 may deal more damage on average, but Dungeon Levels 7–8 are almost as dangerous. The Tangler dishes out the most damage of anything in the game, and Gremlins will steal half of your Food per hit, costing you money even if they don't kill you. The rest of the monsters all do standard "max damage," which can range up as high as 255.
By this point the dungeon Creatures are getting extremely dangerous, and Gremlins will steal half of your Food per hit, costing you Gold even if they don't kill you. They can starve you very quickly, so don't take them lightly.