This achievement is for completing Mission 6 called Evacuation on any difficulty. It is unmissable if you are playing through the campaign.
At the start of the level jump into the Warthog and blast your way through as far as you can. You are playing the reverse of Mission 3 so it should seem familiar.
Towards the end of the mission you will arrive back at the elevator where you originally came down after leaving your Pelican. As the elevator is going up you will fight 3 waves of Prometheans. The final wave consists of 3 Knights and is the only one that gave me any issue on Legendary.
Once you have finished this you will need to run and climb the rest of the way up. You will be getting shot at by Phaetons for a brief while but the last battle is against 5 or 6 Soldiers with a couple of Watchers to keep repairing them. Then its just a straight run to the finish, but keep in mind you will need to run to your Pelican