Tip 1: Make sure to take your hero out of combat mode regularly so that he can charge up more mana to use spells. If you need to cross the map, use that time to recharge a bit. If you're between waves, recharge. It's a pattern you should get used to.
Tip 2: Learn the whirlwind spell as soon as you can. Between it and your hero you should be able to block off two different enemy access points. Then your towers can start picking everyone off.
Tip 3: Study your environments closely. Many include interactive elements that can be used to control (or destroy) waves of enemies.