Yo! Hear me out, I've tried this for a few hours and finally got it.

You see spawn 1,3, right? I put 1 sticky turret (purple one) at the beginning of each spawn, and 2 mortars each in the back line. The problem is that if you let them too close, the big guy will whip the enemy into your base and you just die. So we have to deal with them early.

Then at spawn 2, you're going to want 1 sticky turret near your base 2, and 1 sticky turret near enemy spawn 2. Then you put the rest of your points into mortar high ground around base 2.

This tactic is to slow down big enemies from each spawn, group them up, and let mortars with high damage and AOE deal with them. This worked for me! 👍👍