Guide to get this achievement quickly:
At the beginning, turn left, and jump. Run along this ledge, until you can jump on another ledge to your right. Follow this ledge as far as you can, then turn invisible and run into the building. Run through the next couple rooms, using grenades to clear enemies out of your way. At the end of the building, turn left and grab the ghost that a brute chieftain is getting off of.
Boost through the next several areas until you reach "fight club." Kill the two sniper jackals there. take a beam rifle, and clear out as many brutes as you can. Swap to the other beam rifle and jump down. Advance up the right. More brutes will appear, snipe them or whatever you prefer. Keep moving to avoid the wraiths, and enter the building. Grab the rocket launcher and fuel rod gun.
It's possible to skip the wraiths, sometimes, but you are safer if you take them out. Exit, the building, kill the snipers with melee. Lock on to each waith with the rocket launcher and fire twice. Switch to the fuel rod gun. Use that to clear a group of brutes. Hijack a ghost, and boost to the end.