Use different attacks in battle. Tap and hold the regular attack button to use your standard attack combo, or use special attacks to eliminate enemies faster. Use your special attack as often as possible to kill multiple enemies at once, as well cause lots of splash damage. Consider different types of attacks based on their range. Short range attacks do lots of damage.
Grind previously completed levels when you get stuck on your current level. Use the experience and gold obtained to perform enhancements as well as skill upgrades.