So I guess I power leveled without really trying. I slammed through everything with little to no trouble. The last 3 bosses for instance all fell pretty quickly. The 3rd-to-last in particular. There's apparently an overflow bug glitch on the 3rd-to-last boss that freezes the game for the NA version. If you make it to the 3rd defense stage, the boss does some special move that's bugged and freezes the game so the only way to win the fight is to power level so that you can defeat the boss before that happens or pray that move doesn't get used.

In my case, the boss fell pretty darn fast. I didn't know exactly who the 3rd-to-last boss was, but guessed successfully based on how the wiki mentions you can attack the boss in optional bonus fights where the bug is still present. Anyone who played Ar Tonelico would have guessed similarly I'm sure.

So yeah, beat those bosses and got Mir's ending. I've added the game to my Games Complete list since I'm satisfied. I'm sure I'll sit back and work on it some more from time to time, but I'm not really a perfectionist - plus I have too many freaking games left to play still!!!