Well, even if you leveled up (otherwise, if you win all fights it's true you will probably go up faster, but I think you will lose most of the "interesting" content), you can still use save editor. Just give yourself the belt (if you did not get it the normal way), plenty of money, and before getting in the train, at the kami gate, buy lots of stuff. Or in the editor, give yourself directly lots of bullets, health stuff etc., and during fights, use quite a lot the belt (it gives you instant revitalisation).
It is a pain that the enemy after being defeated and dead, come back immediately with full strength and you don't.
I am sure others will disagree, but for me, that is the programmer (or the enemies) cheating, so, I don't see why I should not cheat getting myself the stuff to fight on
But be aware, even doing like I suggested, it takes lots of time, and it is not a guaranteed victory, you still have to fight.