Destroying a turret in the campaign is easy enough. It doesn't take that many hits before it is blown apart. Turrets fire a rather slow tank shell at enemy vehicles and infantry (though they are more effective on vehicles, since infantry can easily dodge their shots) The easiest way to destroy them is with a heavy tank or a rocket launcher. If those aren't options, then you can simply walk up to one (make sure you dodge their attacks) and plant a C4 on them. Most of the time, in the campaign, they will already be pre-occupied destroying GDI's Offshore Gunboats or other allied vehicles.