I beat him in less than 20 minutes on my first game. Not one of my girls was level 50.

If you keep on meeting Angra at digs since chapter 2, you go chipping away at Angra's HP (he doesn't recover) and getting yourself a nice cache of Megaelixir.
I had 45 Megaelixir pilfered from it. By the time I had the final fight the beast was almost lower than 100,000 HP. A couple of hits was all it took to bring it down.

Digging that much did kill my completion points at Djose though. The first time I fought the experiment the beast was already at level 5

When I played "New Game" and went through FFX2 the second time, I almost didn't dig so it took me a looong time to beat Angra.. I was "WTF ??!?! I beat him so easily the last time ?!?" but then I remembered about the effect of battling him a lot while digging