Coop is the best way trust me. The reason that your cargo has so much damage is because every time you kill a banshee he sacrefice himsef on the cargo. Build hornets and flametrowers thats the best combo. Also put as many marines in the sniper towers, they delay your enemies.
When you start its best that the forge player search for crates and heal him as best if you can his hog does extremely damage against enemies. The other player ( hornet guy ) kills as many enemies as he can find and after that take the bases and defend them immediatly with turrets.
Player 1 has to build and defence the bases
Player 2 has defend the cargo and make troop and send them to player 1
trust me this is the best way, i hope you will succeed.
PS: micro management is the key to victory!!