The videos are a big help but here are 5 helpful tips which should increase your chance of unlocking this achievement (For co-op and solo):
> PRACTISE - Save and quit when you get to the warthog at the start of the countdown. If you fail, you can dashboard and start the escape again. Try to get to Foehammer with at least 2:30:00 on the countdown. If not achieved, dashboard IMMEDIATELY before it can autosave and start again.
> GOOD CONTROL - Drive as straight as you can when going along the narrow parts and try not to stop too much. If you flip you WILL have to start again as this wastes time.
> DON'T USE SHORT-CUT TUNNELS - They will waste precious seconds and take longer than just going the normal route.
> AVOID FLIPPING OVER - Again, this is a must and going for big jumps your're not going to make doesn't help. For the huge jump at the end, fully accelerate and then let go just before the warthog goes over the end of the platform.
> DON'T FIGHT AT THE END - An easy way to bypass the enemies when you have to run to the Longsword is by going along the left side of the left barriers (jumping is quicker), as the covenant kill the flood for you.
By doing this i managed to finish the mission with 1:09 left on the timer.
Hope this helps and good luck!