I don't think the difficulty is based on what characters you have, but what parts of the map you have unlocked. Go to the map, click on the Map Modes button, and switch to the difficulty map. It'll show you which areas of the map are which difficulty.
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Whichever the case, it's starting to feel like I'm just entering a failstate I can't really get out of.
Edit: Guess I just need to make more use of weapons and maybe training. Had some poor starts trying weaker chars in harder dungeons before, but my most recent attempts are doing okay. Might need to identify problem chars and check their stats more closely. Was also neglecting the personal goals a bit too much.