Halo 3

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.

Lately there has been alot of Capture the Flag game types coming up on Heretic in the Team Mythic playlist, so just play a few matches in there and after a few tries, you'll land a CTF. Also vetoing any non CTF maps will help you

The Legendary achievements are also do-able on the Mythic maps. Just join a Mythic game type, complete the achievement, and it will pop up upon completion. Of course after the 5 second (maybe even 10 second) delay.

The way i would do this is by getting a lazer/Sniper and letting the enemy get the flag then puting them in a wide open space where you can esely kill them. next when he/she is in a wide open space Kill them. now let the next person get the flag and then kill them as soon as they pick up the flag. (Bleep bloop)

Note: you can use what ever Weapons you want just make sure they dont drop the flag or get away.

Tip: also try to stay out of close combat, as flags, bombs, or oddball skulls are one hit kill Weapons.

theres nothing really tricky about this achievement.
its called flag dropped, all u need is two flag drop badges and u u got it.
for boosting u just stay at ur base and two enemies grabe the fage and kill the.
if u wanna get this legit ofcourse, just camp at the base and wait till someone grabs it and kill them.
repeat that twice and pop goes the cheevo! lol

This, and all DLC achievements can be done in the Team Mythic playlist (except the Delicious Brains and Zombie Repeller, which must be done in the Infection playlist, Tank Dropper and collecting the skulls, which must be done in Forge mode, or Killtacular which requires 5 opponents, either in a free-for-all or Big Team playlist). All DLC achievements will unlock on maps from later map packs.

Team Mythic requires 6 players, and maxes out at 8 players. You'll have to get 2 players together with 4 controllers each, 8 players, or somewhere in between. If you can't get 8 players and/or guests, you can do this with only 6; however, you may need to put up with randoms, but they will likely be willing to help. After all, who would still be playing this mode unless they were achievement hunting.

You'll need a Capture the Flag game type. Other than that, I think this is a pretty straight forward achievement. When going for this achievement, make sure you get Defend This achievement at the same time.