Step 1: Acquire a good group of 4 people who can listen to directions well and follow them.
Step 2: Load up lost Platoon on Heroic with desired skulls on.
Step 3: Have one person go way in the back behind the hill where the big garage door thing is and chill there hiding. 1 of the others gets the sniper and passenger in the Warthog and another gets the gun on the Warthog while the last gets the spartan laser and takes driver. Then go follow your lone comrade. The Warthog will be doing most of the work while the splaser guy shoots down the wraith when it gets dropped near you.
Step 3.5: There are 3 entrance points for ground troops. Facing the hill there is one entrance ramp on the right, one on the far left and the big path leading down to the door. Air troops can jump or fly in and catch you off guard so be ready.
Step 4: Now the Warthog will be gunning down any and all resistance and attempting to get Brutes out of their choppers. If a Phantom has one that drops close just past the hill across from the big door the sniper should attempt to shoot the driver out before he drops. The 2 people without vehicles should get the choppers when safe enough. Then the other 2 get choppers when they free up. Any choppers from here should be stockpiled in the corner behind the big door along with your Warthog.
Step 4.5: The 4 choppers should be divided among the areas. 2 looking to the left, and 2 to the right while 1 keeps an eye on the open space above the hill to watch for Jump Brutes or Buggers.
Step 5: Kill till you get passed Set 4. MAKE SURE YOU PASS THE GRUNT WAVE AND THE ACHIEVEMENT POPS BEFORE LEAVING!
IMPORTANT INFORMATION:
Chieftains: You can quite easily splatter them if they get too close. Just be careful not to miss because you might get destroyed with a hammer.
Heroes: They aren't too much trouble, just make sure they are taken out before they get close enough to shoot.
Choppers: They may charge you all the way into the back corner but often turn around before they do.
Jump Brutes: These are nasty since they can jump in, stick you, and jump away. Work with your team on how to eliminate them efficiently.
All others: Should be of little to no concern.
Lastly you may want to have one person go out quickly to encourage enemies to move in towards you. Also the back button provides a very nice little map that shows you enemy positions.
Anything else should be worked out among the team.
Btw this will probably take around 5 hours.
Enjoy.