It was surprisingly hard to beat the 3rd dungeon, because you can only boost yourself to a certain power level for the Elements. What I had to do was grind and make a lot of Medium potions, so that you can restore your Element resistance between enemies. Small potions won't heal you enough from my experience. Also be sure to figure out which ability is each enemies weakness so you won't waste potions and Damage per turn. Oh, and definitely save before you attempt the dungeon that way you won't have to redo all the grind if you mess up.
I hope that helps.