***UPDATE!! The latest MCC update around October 12th 2015 has fixed this formerly glitched achievement! You just need to redo the requirements!Solution:
Very easy method! The account going for the achievement will get it even if other players or guests use vehicles/turrets! So, if you have two controllers, you can plug in a guest account, and have your guest account use any and all vehicles! Just make sure you never get in or out of anything with your main account. Note: apparently you can pick up turrets and still get the achievement! (Thanks to TheHunter II7)
You can also combine this if you want with "Ranger", par score for this mission, I collected almost all the points with my guest account(competitive scoring), and the achievement popped for my main account along with "Who Needs Them?".
For this on Legendary, do it your preferred method, or if you don't have a method...
You can try to speed run, or take it slow, I took it slow and killed everything for points.. make sure with Hunters, you circle very carefully if you're going for the in-close kills, and for other parts, save power weapons and give them to your marines! (I had a Brute Prowler with a Fuel Rod Marine on one side, a Rocket Launcher Marine on the other side, and a 3rd in the gunner. Just make sure to drive circles so both side gunners can blow stuff up! Ghosts are your nemesis at all times, they are primary targets. When you get the tank, kill everything from a long distance. And for fighting the Scarab, use a Gauss Warthog otherwise the Scarab will take you out.
Hope this helps!