I found one earlier in the game, in Bermia Gorge, but it beat me pretty fast? I know that if you beat them you can get a really powerful weapon for each character.

Is there anyway to find them the later at the end of the game; or if you miss one you need to try it on the next play trough...

You can find all of them at any point when you can fast travel around the world so you don't need to worry about beating them until later. You'll want to beat them on your first playthrough, as you'll receive an item that is only useful during your second. If you reach the end and are still looking for ways to power level, there's a pretty handy optional dungeon near the Dimensional Breach

Good luck!

I'll try that, at the moment my level is around 50 and I just beat Alvin.. I guess I'll wait towards close to the final boss.

Yeah pretty much do it as the last thing. I think you should be at least level 55 before you attempt them.

that one in Bermia Gorge made me angry, lol. stupid harpy kept beating my by constantly confusing 2-3 members of my party at a time. finally got my party to 100% light resist and took her down.