You win this achievement by playing the 5th level: Tip of the Spear. This is the level that starts out with the huge battle scene and everyone in warthogs.
A couple of pointers on this...(besides the always "Work slow, take your time")
1) Take out the turrents 1st and formost whenever you see one. These things will just own you...so sneaking to the side and pick the guy off is a must. The AI WILL hop in and fill in the spot, so once you clear it, watch it for the next 20 seconds and clear out anyone that hops in it.
2) take the warthog into the center and clear out all the enemies. Your mission is to get to the tower and disable it...we threw grenades into the main structure and ~2 take it out, you can also shoot it if you have an AR. Hop back in the Warthog and continue with it until your cross the bridge. And clear through the building (see extra achievement below)
3) Keep in your ghost as long as you can. You will come to a vast open area, and using that to get around and pick off players is a must.
[Part two of the video, check the youtube page]
4) In the helicopter station, if you are Co-Op, only one player needs to survive. Just prioritize, the gunners [co-op only] need to hit the people, the grenadiers need to hit the turrets. Work QUICKLY through them and aim as far up the canon as you can.
PROTIP: If you hold the trigger the grenade with only explode once you let it go, notice the arrow that cycles around your sights, turning yellow once it reaches the right side...this also releases an EMP blast. you are better off with short taps of the trigger so they will explode shortly after contact with the ground.
5) Continue through the level until you reach the large building at the end. In co-op, have one person stand on the bottom level right by the teleport up. On the top level, take advantage of your grenades, and work around quickly. In the video it shows him easily killing the Elite w/ Sword...but in ours he just stood there with a box between me and him, and I put 150+ rounds of my AR into his head and his shields would instantly charge...we finally beat him because he accidentally flew off the side of the tower.
Also, if you work quickly enough, this achievement is available about 3/4 of the way through the level. [@11:15 of part one is when you see him in the video]