In order to earn this trophy you need to complete a dungeon level without ever losing any hit points (taking damage). This can be done while revisiting an old area.
Thus it is very simple to get this on the first dungeon (Ancient Temple) if you use well-leveled CPU-controlled allies. Just proceed through the level cautiously, letting your allies do the killing. Be careful of the ranged attacks of the Harpy boss these can hit you from almost all the way across the screen. I did this easily with merely level 6-7 allies, while a level 3 sorceress, so you can get it fairly early in the game. The trophy will award on the results screen after killing the boss (assuming you took no damage).
Note: After you finish all of the nine dungeons once they will all get slightly harder (even the A routes get extra enemies) and the bosses lvl will increase. This Trophy will be easier to get with a character who has not yet completed the ninth mission (Lost Woods).