A good, effective way to level up is to farm exp eggs from harpis which are located near kohlingten with the ragnarok esper (3 to 4 of those should do), train in the dinosaur forest using the vanish/x-zone method if you're playing the snes/psx versions and that will rack up your levels pretty quickly. Also make sure to equip espers that raise your stats with each level and make sure to save your game with the stat buffs otherwise you will lose the amount of stat buffs you've obtained through leveling (they reset in the snes version)