This game is WAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAYYYYYYYYY to difficult man, the damage/speed of the towers compared to the enemies makes them impossible to kill. I kept trying over and over again to see if their was a good strategy, but their isn't. Also, after a while, the game starts to glitch and spawns all of them at the same time...
Enemies move a little fast for the towers.. Try to even it out more power from the towers or slow the enemies down.