Insanely hard if you try to rush through too quickly. Get rid of all of your opponents to proceed to the next floor.
Incidentally, these are a good floor to power-level lower level characters, provided they bought decent gear from your shop prior to using them, or if you're willing to let them borrow good gear from your inventory prior to coming here. Floors with bosses can sometimes prove a bit challenging for a low-level adventurer to survive in, but these floors simply involve killing a lot of regular enemies at the end.
Amber Garden: 15F
Obsidian Tower: 25F, 35F,
Lapis Ruins: 30F, 45F, 65F, 75F, 85F, 95F