The Legendary maps are Ghost Town, Blackout, and Avalanche.
This is pretty much the exact opposite of the "Defend This!" achievement. Instead of killing someone with the flag as a weapon, you have to kill someone carrying the flag. Twice.
You're best bet is to play DLC BTB and hope you get a CTF gametype on Avalanche (as the other Legendary maps tend to not have CTF as a playable gametype on the playlists). CTF doesn't pop up too often for some reason, so it could take a bit to get this achievement.
When the game starts, you're going to want to hang back by the base and keep an eye on your flag. Hopefully there will be enough people on the other team trying to win the game, so they'll go after the flag (Most of the time, it's people driving around in vehicles trying to get a splatter spree of Warthog chaingun kills).
When someone captures your flag, you must kill them. Kill them anyway you please but it's best to try doing it from some sort of distance as the flag is a one-hit kill weapon. Use ther flag carrier's slower running to your advantage. Throw grenades, AR, BR, sniper, needle...Or assassinate if you can. Whatever will net you the kill: do it.
Now, you have to do this twice in one match. CTF matches last a long time anyway, so time shouldn't be an issue. You should unlock the achievement shortly after the second kill.