Later in the game you will incounter treasure chest's during battle. Some of these chests have nothing in them and as a result you have to fight a powerful mimic. But there is away around this. First make sure Rikku (or anyone) has the ability "Mug." When in a battle with a chest, use mug on the chest, if the chest has an item, you get it and kill the chest. If not, you get no item and kill the chest. Since chest's have 1 HP this is a great way to avoid mimics.