it's possible it just takes a looooong time and a lot of stages with powerful monsters. If you want a hint use the darkworld A LOT, and use the land of carnage about ten times more.
I got the main character up to level 834 after reincarnating him several times and not doing it again until he was at lvl 800 again, and he was a bad ass, but there were still quite a few super bosses that just whooped me! I loved it.
It's one of my favorite PS2 RPGs. The story isn't that great, but the ending is awesome simply because it was extremely obvious and I didn't see it coming(nor did my friend). And the characters are funny as hell. Plus, the game has so freaking much more to do than just the main story, I still haven't done everything in it, then again, I haven't played it in a while.