Okay, to be honest, I have no idea how any one could beat that. :D
But here is what I would do.
Push a couple of marines out to the right. Then back track, retrieve the sniper rifle ammo, throw a frag grenade to knock out a few of the flanking brutes. Throwing my final grenade into the ten other brutes. Then snipe the brute driver out of one of the prowlers. Throw the plasma grenade and stick the other prowler. Move to my right, letting the marines and ODSTs cover me. I drop the BR and pick up the rocket launcher. Then I fire both rockets into the wraith. Then on the second pair, I would put one in the remaining prowler, then finish off the survivors of my frag grenades with the second. Drop the now empty RL and pick up the BR, then pick off the final brutes that had been flanking me. Sprint to the gauss hog and blast the incoming phantoms out of the sky, taking the grunt reinforcements with them.
It might not work, all of it is very risky.