Find an area where the mantis-like creatures appear. Use the Paradise Apple: Body skill as you fight. Leave two of your enemies alive on the field so that they will occasionally call for reinforcements. Keep killing any new troops that arrive, but leave the initial two enemies alive. The longer the battle lasts, the more your experience points will increase. You can repeat with Paradise Apple: Skill and Wins to earn large amounts of SP and OZ. It's the fastest way to level up in the game.