How I did this:
Go into Deathmatch (2v2) as Captian Cutter. When the game starts, spawn one barracks and the rest supply depos (turrets if you want). Spawn and many marines as possible out of the barracks and 3 Spartans. Capture any free bases around yours and after you capture each one, start spawning more marines. Also, in any captured base, have one barracks and the rest supply depos also. Go into battle with all of your marines BUT MAKE SURE ALL POWERS ARE UPGRADED FIRST (it will do this automatically after like 3 minutes). Every time a squad of marines dies in battle, use the ODST drop to fill in for them. Your soldiers should eventually make your enemy forces retreat, and when they do, follow them to their base and use all 4 MAC blasts on it and have your soldiers kill the rest of it.
Doesn't always work, but it works about 70% of the time.