Hey, you know that part of Halo 3, on Tsavo highway where you're up on a hill with some little buildings and Katie Sackhoff and wave after wave of pissed off Brutes come charging up the hill and all you have to defend yourself with is a woefully inaccurate mounted machine gun, panicky marines, and some prox mines (well, there's a sniper rifle too, but that might as well be a rubber band gun in my hands); and some of those aforementioned Brutes have fuel rod cannons and then finally after killing a million of them, a Wraith shows up and blows you to hell? Right, that part.
I figured out a way around it.
Run down the hill, kill the guy in back with the fuel rod gun (he won't turn around until you shoot him the first time, 3 shotgun blasts will do it), if you're lucky, the other Brutes won't notice (they're a bit busy), grab the now available fuel rod and knock the gun off the Phantom that's dropping off another hundred or so Brutes and run around the corner.
Look who it is! Its the Wraith that's not scheduled to appear until after you kill those Brutes. (my first idea at this point was to steal that and use it to kill the Brutes, that didn't work out)
Unfortunately there's a sniper tower next to the Wraith tank, but fortunately you have a fuel rod cannon, unfortunately you have to take out the tower while the tank shoots at you, but fortunately you can stay out of range easily enough. After that, kill the Wraith's gunner, and run up and pound on it, thus taking it out.
You will be rewarded with a cacophony of noises as every scripted event for this section fires at once. Johnson tells you that Covenant armor is on the way (you just blew it up), Starbuck announces that more Brutes are coming, Johnson congratulates you, Keyes says that she's coming in with reinforcements. While this is happening, Pelicans and Phantoms are popping in and out of existence above your head.
And there you go, way easier than fighting several thousand Brutes.