You need the Mystery Egg from the Mushroom guy in Moleville, and the B-tub Ring you can buy in Marrymore. Equip the B-tub Ring on the Princess have her use the Mystery Egg in battle until it turns into another item. Then use this new item in another battle until it turns into the Lamb's Lure, which is the sheep attack you must be talking about -- it'll never use up no matter how many times you use it, and it can turn enemies into sheep, which then run away from battle.
Quasar