The Legendary Maps are Ghost Town, Blackout, and Avalanche. The goal here is to get 5 kills with the Warthog's chaingun in one match.
The achievement may as well have said, "Get this achievement on Avalanche," because as far as I know there isn't Warthogs on Ghost Town (and there aren't any Warthogs on Blackout). Pretty much, keep playing the DLC playlists until Avalanche pops up.
Now, hop on the turret for the Warthog. If it's a team game, hopefully someone will drive you around (and hopefully they're good at doing so). If it's a free-for-all, then you're a sitting duck.
Note, though, that these 5 kills don't have to be in a row. So if you get killed, feel free to continue with your chaingun massacre. Another note: if by the powers that be a Warthog with a Gauss cannon is on the map, don't expect the kills from it to count. The achievement specifically says "chaingun."
This is a pretty easy achievement to get. Hopefully you won't have to go through the motions twice, like I had to. I got 5 chaingun kills once before and I never got the achievement. Worked the second time, though.