Be humans on the skirmish lvl with the protecter units and make spartans and some type of vehicle.
go to the protector plant and get a shield generator on the spartan, and then one on the vehicle of your choice.(i use grizzlys)
Next put the spartan in the vehicle by pressing "y" and you have a double strength shield on a vehicle. you can only do this 3 times because you can only make 3 spartans. also the vehicle is super strong because adding a spartan makes it better anyway.