@Diciannove44 If memory serves it was a certain hentai with a "cheating" tag. And that art style reminds me of Distance, but I could be wrong. So there you go. o/ Start your search there. I remember reading it once, but I don't remember the rest.
Picking an easy dungeon to start with was a smart move. They need to not only get used to fighting together, but come to trust each other in order to stand a chance against the truly powerful foes.