Toy Soldiers

On the very last level for the DLC pack the very last wave is this boss.

He's a very simple boss on easy, and you only need to have every large emplacement set up as a Level-3 Howitzer to easily defeat him.

Make sure you take out the enemy howitzers at the beginning with your bomber plane and take their spots.

You will need to rotate the Howitzers every now and then as the boss is moving. Should only take a minute or two to defeat him if you have every spot with a Level-3 Howitzer firing at him.

There are two phases to the boss, but I did not have any trouble with either. Just keep firing with the Howitzers and you will have this achievement in no time!

Edit: Thanks to CanadaAckis for the correction below.

There's actually three phases:

1) Normal, comes at you slowly. Ends when it's out of health.
2) Heals up to about 50%. You have to target the guns on each side, they're flashing red. Two guns on each side.
3) Heals up about 25% and starts going fast towards your base.