Put your abilities to good use. Use Lighting Strike when enemies are clustered together. Teleport will send enemies back a short distance to keep them inside kill zones; MacHellman does good damage and is an invincible friendly unit. Enemies will target him but never bring him down, making him an ideal distraction.
Easy "Boo!" achievement
You actually only have to scare at least fifty enemies in a mission. Consider doing this on "buried paths" because of the totem.
Landmarks
Every level has a landmark that should be used to your advantage. Strategically place towers around these chokepoints to maximize their damage. Some of these landmarks must be activated manually by spending gold.
Towers
Your towers can do physical and magical damage. Refer to the library to see which type is more effective against a particular enemy. When a new enemy appears, tap the small exclamation mark at the top right of the screen to view their information. Below their health will be a red or blue shield and the words "none", "low", "average", or "high". The red shield represents physical resistance and the blue shield is magic resistance. Its description shows its amount of resistance. If an enemy has high magic resistance, use physical damage towers and vice versa. Position slow and powerful towers near disabling towers. For example, partner the Apprentice tower which does large amount of magic damage but has a long cooldown with the Sleeping Lizard tower, which slows enemies.