While they closed one of the Reach Wraith-stealing tricks, they apparently didn't close the other.
To do this, you need to:
- Kill the gunner of an enemy Wraith
- (Optional) Stun the Wraith with an overcharged plasma pistol shot, which keeps it from moving and firing its cannon
- Get in the gunner's seat of that Wraith
- Wait, and the elite driving the Wraith will get out
- Kill him (ideally without leaving the gunner's seat)
- When safe, jump out of the gunner's seat, and jump into the driver's seat
You can do this in Campaign and Spartan Ops, it works fine in either mode.
With regards to the related achievement (Mortardom), Here's a video guide, courtesy of IGN:
There are a lot of Wraiths in the section of the level once you've met the Composer, although at that point there will be friendly Rhinos competing for kills. The easy way to deal with this is to ignore the unoccupied Rhino, and ask the Marine in the other Rhino to hop out and take his. The rocket Mongoose can be somewhat of a pain, so you may wish to arrange for an "accident" (or just take his rocket launcher). Nuke the ghosts and then you can take on the Wraith. It's a bit tricky to manage all of this and get a plasma pistol, so you might consider bringing one from earlier in the mission.
You can also (with some effort and potentially some careful Wraith damage) get the first Wraith (where you grab the first target locator from the downed Pelican) to fit in the Mammoth and drag it around to the various objectives, if you are so inclined. Just punch the little "wingtips" on either side until they snap off. This takes a good number of punches. Be careful about wrangling the Wraith into the Mammoth, especially once the Mammoth starts to move. The Wraith has a tendency to disobey the laws of physics and clip through the Mammoth. It will also despawn when you get to the point where you have to go it alone on foot.
Keep in mind that the Mammoth has sniper rifles on the outer deck, which can make taking gunners down easier. A BR/DMR/Carbine works OK, as long as you're not getting shot at too much from the Wraith or any other unfriendlies in the area.
During my solo Legendary run, I also ran into a glitch when the "unidentified Covenant cruiser" appears where Wraiths and Ghosts kept spawning without moving the mission progress forward. This was an easy place to get Wraith kills, although I'm not particularly certain how I caused this glitch or how I ended it once it had started.
I took the Gauss Warthog that shows up in this area and parked it in the Mammoth, and was getting in and out of it repeatedly, which seemed to cause checkpoints often. Phantoms kept dropping Ghosts and Wraiths. I've gotten the game to do this twice now, so it may be worth a shot if you're having issues getting the kills you need.