I played on normal and had no problem with the first dungeon. Infact, the entire game is pretty easy.
The very first dungeon when you're by yourself is better because you can switch back to Oliver when you're about to get attacked. When more enemies and allies get added, you don't know who is going to be attacked, without going into all out defense it's pointless. Switching to all out defense is a waste on normal battles because instead of taking one attack to one character, you're defending one attack and making all three characters useless for a small period of time. Not efficient to defend on normal battles with more than one character but early in the game it's a lot better than later.
If you want a good action RPG on the PS3, play Tales of Graces F. There's multiple difficulties on that and can be changed at any time.
