First thing is first, get together three of your mates so that you can begin a four-player co-op game. Set your difficulty to Legendary of course, and play the final level in the campaign 'Coastal Highway'. Be sure to also turn on the Iron skull (this causes the entire team to reset to a checkpoint whenever a person dies).
This is going to take a lot of patience, lol.
Best way is to keep on keeping on with your four players, who I recommend be ably skilled at Halo. Be sure not to run and gun into heated fights with brutes, and vehicles.
For the tip that'll make it all that much easier, once you reach the highway, don't take the Warthogs (or Scorpions) obviously and proceed on foot. In the middle of the highway up ahead is an off-ramp, take this down and you'll find a nice cache of two Mongoose and four rocket launchers with a ludicrous amount of ammo (998 in reserve, to be exact).
Now there's only one thing to do, push on and try your hardest. Be patient and just try to milk checkpoints cause it'll make this all that much easier.
As noted above, Hogs and Scorpions are a no-no, though if you stumble across an abandoned Ghost or perhaps the aforementioned stashed Mongeese, then you're entitled to those. So good luck and hopefully this has been marginally helpful.