So I found Co-op mode to be way too easy, so we came up with better rules to make it more challenging:

1) Use all the tiles, no matter how many players you have. Put the Stairs randomly in the bottom half of both the first and second level stacks.
2) You do not earn treasure cards for killing monsters. When you kill a monster all you get is XP....or after you level up 4 times, then instead of additional experience, you can take 1 coin for each monster killed.

Cards are too powerful if you get them one per kill. This method is very difficult.