Instructions
Use Professor Chaos, Pip, Craig and either Kyle or Cartman as your four characters. This group will give you the best advantage against the waves of enemies, as they have the best combination of attack and speed.
Build towers designed to take out waves quickly. This means that you'll want to change a few towers each wave, depending on the enemy that is approaching. For instance, when gnomes are approaching you'll want to have a "Laser." When Mongolians approach, you'll want to replace your "Laser" with a "Cherry Bomb." Constantly switch towers based on who is attacking you.
Use your characters' special attacks in the later stages when your towers can't take out the enemies fast enough. Try to avoid using special attacks on the earlier stages, as they take awhile to build up.