Objective: Survive 5 waves
Focus towers are good. They are able to destroy the most heavily armoured enemy. The longer a focus tower attacks an enemy, the more damage the focus tower inflicts.
One trick in this level us to use slow towers to ensure that the enemies are moving slowly enough so the focus towers are able to do their work. Slow effects wear off, so place them at strategic points to they are able to hit enemies twice on the circuit.
Add a couple of splash towers, take out the pink blobs and upgrade your focus towers once. Easy.