I leveled up by going to a place with a save point and enemies that give a lot of EXP, and by equipping a Lure Bracer. That way I could get into battles all the time, and by staying next to the save point, you can save and restore all your HP/MP after every battle if you want. Where are you in the game? If you're on Chapter 5 and you aren't strong enough for Via Infinito yet, you may want to try out the cave in the Thunder Plains or the Way to the Farplane for leveling up.