When attack a base or deposit the 1st thing you do is takeout the defensive towers. For a machine gun turret you can use a variety of things. Among them are: large groups of heavy gunners/ riflemen, LVL 3 or higher razor backs, LVL 5 or higher cobras, or a platoon of tanks. Once you have taken the gun turrets you can send in air ( cobras & bombers) to take out mortar before deploying tanks to help finish of whatever you are attacking
Send in tanks (Rhino, Paladin, Challenger and Mega tank) to take down a gun turret. If you have not unlocked any tanks you can use rocket launcher, flame thrower and suicide bomber. Razorback are useful if the gun turret has a shorter range then it.