Monster rooms, monster houses--- there were a few different names for them, but they were a huge danger in dungeons. Take one step into one, and find the whole place filled with enemies... If you were unlucky and it was a big chamber, that was pretty much it for you. And neither Nico nor Joli had wide-area attacks for dealing with that sort of thing.
Honestly, in those situations, escape was usually the best solution. But even then, escaping from that many foes was tricky.