Ok, I found this to be the trickiest one so level knowledge will help a lot.
As soon as you drop from the pelican leg it past the first lot into the cave to the AA gun area. Use the rocks on the left to jump up and walk through the door to the area with the downed ship. I recommend using your sniper to headshot the brute between you and the mongoose, then hop in that and keep driving, trying to avoid the prowlers, choppers and ghosts the best you can. If you have an opportunity to, try to hijack a chopper or keep going, eventually you come to an abbandoned one you can knick. Once in a chopper head straight to the area with the AA wraiths and take them out with the chopper as well as any other enemies.
Now this is important, once Forward unto Dawn lands, you MUST wait until the screen says forward unto dawn and the scorpians start to be dropped. After that its safe to keep driving the chooper into the area where Guilty Spark will open the door. Once in this area you MUST take out the three wraiths in this area. Once thats done head up to the door and kill any stragglers, then Spark should open the door.
Head through and activate the bridge and carry on. When the scarab heads over keep walking and slide down the cliff to the scorpions. Grab one of them and keep heading down to where the scarab comes back taking out what you can. When the scarab comes down focus on one leg, then when its downed either hop on and take it out or simply drive to the back and shoot at the power core with the scorpion.
Once the scarab is dead head to the ramp and make sure you kill EVERYTHING on the way up the spire, otherwise the pelican wont drop off the arbiter or the marines.
Once you're in the room with the sleeping grunts just keep running through until you reach the cartographer. Then head back though and take out the brutes and the cheiften. Head down the slope to where the cloaked brutes are and jump down. Hopefully you'll catch up to the hammer cheiften and assassinate him for a quick kill. Johnson should then say hes coming to pick you up, meanwhile kill EVERYTHING IN THIS AREA! That should hopefully end the level and nab yourself the achievement. I managed it in 17 minutes.
Good luck ( and sorry for all the reading!!)