Max out an Anti-Tank Turrets and put 2 Corrosive Turrets behind. The Anti-Tank Turret will act as a shield, and the Corrosive Turrets will finish the Decimator quickly.
Just use a bunch of Heavyweight Turrets and Tesla Towers with range-increasing parts on them, have one of them deliver concussion effect while the other deals lots of damage. This works on EVERYTHING.
The Corrosive Turrets best at Lvl. 10 or higher and the Anti-Tank Turrets at Lvl. 7 or higher. They have a good ammount of Health. If your Engineering Lab is high enough, you can add parts to your Corrosive and Anti-Tank Turrets. Make sure these parts have LOTS of Health and Damage boosts (If nessesary range increasing). If you can add Splash and Concussion Effect, that wouldn't be bad either.