You have different elements of the earth, air, water, and fire to destroy all of the enemies in each level. It could be fire, lightning bolts, wind and more that you might need to use. Each level is set up as a kind of puzzle. It is up to you to decide what you need to make sure each enemy is dead at the end of a level. You will fail the level if you haven't destroyed them all. As you go on there will be more and more enemies to destroy and you will unlock more elements to help you.