Toy Soldiers

This is real easy to get and can be one of the first achievements to obtain.

Pretty much the first three things you start the game with is a machine gun turret, barbed wire fencing and a type of mortar. Simply spawn a mortar at any location and hold the LT and RT buttons down - this will allow you to control the mortar shell slightly and will give you a camera angle from behind it.

Simply guide it until it hits some enemies and the achievement should pop up!

While using a Howitzer or mortar press the right trigger to fire. Hold the right trigger to guide the shell with the left directional stick. Guide a shell by holding the right trigger to gain this achievement.

you could also get this by upgrading the machine gun to a cannon and when you would normally just press the trigger to fire instead hold down the fire button.