If you are having too much difficulty passing a level, wait until you are about to die then save your game and quit. Restart that game and you will start back at the beginning with full health, full spirit, the experience you just gained, and all enemies are re-spawned for you to kill again. Repeat this several times to gain easy level ups, gems for buying power-ups and items, and to rack up kills in order to reach the 500 point skill increases.