There are certain elemental attacks that will alter the base element of the enemy that you are fighting so you may need to change your tactics to compensate for this and be more effective.
Gain Levels Quickly
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.
Sopia
Always equip any Sopia you get from other characters as it allows you to gain an Element. Sopia can by upgraded to unlock new abilities using SP earned in battle.