If you want to do this without using any cheat skulls or anything, here's the best method.
Definitely do this on Heroic; Legendary is near impossible.
The hardest part is the beginning of the bridge, as there's no checkpoint until you get halfway across. What you want to do is get the Marine with the Rocket Launcher to take out the Ghosts quick and easy. Just lure the Ghosts to his attention and he'll fire away with abandon. You also want to try and keep the other Marine alive, so that if the Rocket Marine dies, you can give his Rocket Launcher to her.
Things to be careful of:
- Don't let yourself be killed by a rocket!
- Sometimes the Marine will be behind a low car and shoot his rocket at the car, killing him.
- If the Ghosts target either of the Marines, chances are the Marines will die, most likely from being splattered.
The Wraith tank is a little tough. Make sure all the Ghosts are dead, and grab the Rocket Launcher yourself and kill it (make sure to leave at least one rocket!). Use grenades when you have one rocket left.
If the original Marines are dead, that's fine, because at this point there will be two more to your left: one with another Rocket Launcher, and one with a sniper. Give the Sniper Marine the old Rocket Launcher, and now you have twice the firepower!
You'll get plenty of Checkpoints between this part and the end, so don't hesitate to Revert to Last Checkpoint if one of the Marines dies.
Watch out for the three Phantoms and two Banshees coming from behind you. You'll want the Marines to shoot the Phantom carrying the Wraith so that the Wraith falls and you don't have to deal with it later.
There will be a few more waves of Ghosts advancing towards you, but with the two Rocket Marines, they're easy to handle.
Once you get towards the end, Cortana will say "That's quite a welcome party," and three Banshees will arrive. These guys are a pain, because the only weapon effective against them is the Rocket Launcher, and the Marines will have trouble targeting them, since the Banshees float around far in the distance until you get within a certain range. Try to lure one at a time, because handling more than one is very difficult.
The Marines will have awkward pathing at this point. One will usually sit behind a barrier on the left side of the bridge, and the other may or may not advance forward on the right side, leaving him out in the open to get killed by a Banshee. Try to keep at least one of them alive until all the banshees are dead, as the Banshees can take one or two rockets each to kill.
The Banshees themselves will shoot at you from quite a distance with accurate aim and tracking, so dodge to the best of your abilities. If you get behind cover, though, they'll stop targeting you and either fly away or target one of the Marines, who likely won't be targeting the Banshee.
So find a good balance between dodging, going to cover, and making sure they don't target the Marines but the Marines target them. Once one or two of the Marines hit a Banshee, it's dead.
This part may take several tries, but once done, you're in the home stretch. Make sure to save some rockets, though.
The final hurdle is dealing with the two Wraith tanks that come out of the tunnel. If both Marines are dead, that's fine. Use everything you have on these two Wraiths until they're dead. Make sure to avoid their fire and plant grenades effectively.
After that, go down into the tunnel. The rest of the mission is easy, as unlike the first part, you're provided ample supplies to proceed on foot. Just don't board any Ghosts or get in any Warthogs and you'll be good.