If you want to win without cheating:
First, get to at least lvl 8 but DO NOT complete the tenth floor.
Second, buy the expensive health and magic potions at least 10 each. Make sure it's the one from the Apothecary and not the convenient store. The ones that say "Battle Item".
Third, take out the guy on the left first with Rasengan spam while keeping an eye on how much HP they take. Make sure you heal when you're near that amount and drink magic potion
Fourth, take out the guy on the right. He's a pain but not as much as the one on the left. Rasengan spam of course.
Finally, take out the leader. You know how.