my method of leveling up is a cop out in a way, but what i did was when i had odin was go to the island closest to hell and as soon as the battle starts have my fingers on the escape shoulder buttons. every so often odin shows up and kills the creature, gets you to level 100 easy peasy lemon squeezy pie!!!!!!
I find it a lot easier just to fight the monsters.

not to mention a LOT quicker.