A tip for collecting them all: if you start on a level and it won't have anything new, press ESC and select a new character. The level re-randomizes. Doesn't help if you need to finish levels to see what turns up, but it's useful if you're only looking for specific missing monsters found on only one level (i.e. bosses or the weirdo ones like Santa).
 "Great Dungeon in the Sky"){kind=icon}