I found this very easy on the second mission, "A New Enemy". I gave my teammate control of all the flamethrowers and he focused on taking out the supply buildings and other stuff for the bonus objectives.
I found the key to this mission was leading every attack with the Spartan, and getting Marines into all the sniper towers ASAP. Use the Spartan and Warthogs to chase away large Banished armies, and claim the sniper towers quickly.
Once you've got Marines in those towers, the Banished try to attack but usually run away. Just keep claiming towers and taking out Banished stuff(including the 2nd Banished base so you can build another) and roll up the map.
We finished in about 28 minutes, 8 minutes over par time, with a score of 147,000. Gold medal score is I believe 90,000.
