Strategies For Short

Wave 11: Make sure you get hidden support as soon as you can if you don't. These hidden surely will screw you over.

Wave 9: The boss has a lot of HP get good Towers (For example: Archer, M249 Turret, Tripod Ravager, etc.)

Wave 6: These slows have a low amount of health but can easily destroy you in big numbers. You might wanna get fast firerate towers.

Wave 27: Fire is very good for slowing the guardians. Level 4 inferno tower can also kill them easily.