This achievement is fairly straight forward. It will, however, be somewhat tedious and frustrating.
Organize a campaign lobby with 3 of your friends in Halo 3 campaign and select the final Mission "Halo". Make sure to set the Iron Skull on. Proceed through the mission with your friends as you usually would, but have everyone stay reasonably close together and constantly support each other. This is because the Iron Skull sends you back to the last checkpoint if even one of your team members die. Stand a bit back and kill all the Flood in the area before proceeding forward.
Early on in this mission you will see some Flood Stalker forms climbing the installation. Try to kill at least some of them as it will make holding off the Flood before entering the control room a bit easier. To make this part a lot easier, there is a trick jump everyone can do to hide on the side of the cliff rather than be among the horde of Flood. All you have to do is jump up the side of the door and use the indentations to climb up to the cliff with Johnson. If this isn't clear then check the video at the bottom of the guide.
Keep going through the mission until you've
and head back outside. Instead of going up the newly formed ice-path, head back down one ramp and look under it: BAM! 4 Ghosts for you and your buddies. Take these ghosts through the rest of the mission. Once you've gone over the ice-path you should come to a metallic opening: get off your ghosts and clear out some of the Flood inside as to avoid unnecessary damage to your ghosts. Make sure not to head too far in while off your ghost
, otherwise it will load further and will result in a glitch that does not allow you to go back outside to grab your ghost.
Get to the final driving section of the mission. Ignore the Warthog, just continue on your Ghosts. Have someone who knows the final driving section well to lead the group while everyone follows behind. Essentially you want a straight line of four Ghosts the whole way. If someone's Ghost falls off the edge, restart the checkpoint immediately
, If you don't it may checkpoint you and the Ghost it lost forever and the you'll have to start the whole mission again.
Once the person at the head of the line reaches UNSC Forward Unto Dawn, and everyone is driving their Ghost
, the mission will end and the achievement should pop.
If you're not sure about anything I said be sure to ask me a question or watch this guide by Achievement Hunter.