I just managed to get this achievement and did it with a minimum of fuss. You obtain the Scorpion shortly after the first "engagement" area of Level 9. In this first area, there are two Shade Turrets, two Ghosts, several Grunts with Fuel Rod Cannons, and a Jackal or two with beam rifles. Dispatching these initial enemies can take a bit of time, but once killed, I would highly recommend hanging on to your DMR and a Fuel Rod Cannon, they will make your life MUCH easier.
The best solution I have is to take it slow. Upon getting in to the Scorpion, take your time and fire from a distance. Most of the initial enemies can be taken down quite easily and you barely absorb any damage.
As you approach the AA guns, you will climb a small ridge. Before getting over the top, simply begin firing at the first gun. After a few shots, you'll start to see the flame plumage occur, indicating that the AA gun is taking damage. A few more shots finishes them off, as well as any enemies that were inside. Repeat this process for the second AA gun.
After the AA guns are taken out, several Banshees begin to fly in your general area. Again, stay back, take it slow, and take them out one-by-one.
Once the Banshees are dispatched, you roll up a short lane that begins to rise into a hill. At the top of the hill there is a levitating sniper platform (easily taken out by your Scorpion) and a few Ghosts with pilots (Grunts and Elites, I believe). Again, go slow, and take out the Ghosts one by one so that they don't gang up on you.
After clearing the small encampment, the REAL challenge begins.
Directly after the area with the sniper platform and Ghosts, there is a massive killzone area, featuring several Shade turrets (both heavy ones that fire Fuel Cannon Rods and smaller ones which fire plasma rifles) as well as few Ghosts and a few Revenants. These enemies WILL toast you if you are not careful.
Like many of the other solution artists, I found the best way to deal with these enemies is to hop out of the Scorpion and scout targets on foot using the scope on my DMR. Upon pinpointing where the target is located, hop back in the Scorpion, carefully line up you shot, and blow it away. The only enemy that I had problems with was the Fuel Rod Cannon Shade that is mounted high and to the left as you round the corner. I managed to take this out by plugging away at it with my DMR and launching a few Fuel Rods at it from my Fuel Rod Cannon.
(Quick "safety-blanket" here. If you do die in this kill zone, you will typically respawn with the Scorpion just before the previous area with the Sniper platform and Ghosts... just be conscious of when you hit checkpoints.)
After clearing out this very hot kill zone, begin to roll up to the door of Sword Base, but be sure to take out the final Revenant as you approach.
Good Luck!