The first thing is to group all your units and move as fast as possible because the longer it will take to get to the freaker base the more units they will build. Then get over the bridge and kill a couple of dire wolves. Don't go directly to the south since these grapeshot towers are impossible to attack from the narrow bridge.
Instead, go north, destroy a monster truck and turn to west. Go to the edge of the map and turn to the south. Kill a hidden scorpion and a little bit south you will see a gun nest and oil rig. Destroy them as quickly as possible. Then move down to the southern edge of the map and go closer to the edge to east destroying everything you can reach but keep close to the edge or else these grapeshot tower would blow you up. Destroy the jail and group all units together.
Now it's time for towers. Attack them one-by-one so that while you are attacking one, another cannot hit your units. Also, it is better to wait until your veteran units can heal themselves. Try to keep alive as many units as possible since you would have to deal with two machine gun nests after these towers. This can be easily done by fast attack followed by retreat to give time to veteran units to heal (there is nobody who can repair these towers!). Be very accurate! By the end of the mission you would have probably only 3 or 4 units left.