Halo 3

The Legendary maps are Ghost Town, Blackout, and Avalanche.

I haven't play a CTF game on ranked or social setlists that weren't on Avalanche. To help, I'd suggest playing on the DLC BTB playlist. However, CTF seems to come up very rarely.

Anyway....if you happen to be playing on Avalanche and it's a CTF game type, you're in luck! You're goal is to acquire the flag and kill someone with it. I would suggest taking the teleporter from your base to the mancannon by the outside edge of the map. Use it to get to the middle and move towards the other mancannon straight ahead (you're going to be going to the other teams teleporter room to get inside of their base).
Quickly grab the flag (crouch walk if you got to) and make a run for the teleporter. Once you go through the teleporter, move to the side and wait. Chances are, at least one guy will follow you. When they come out and walk past you, hit them with the flag. It should be an instant kill and you should get your achievement a few seconds after the kill has been made.

So, to summarize (CTF on avalanche)
1) Teleport from your base to mancannon
2) Mancannon to middle
3) Walk/use ghost to other team's side mancannon
4) Use the teleporter
4) Grab flag and go through teleporter
5) Move to the side and camp
6) Assassinate anyone who follows you.

Obviously you can get this kill any other way, since the flag is an instant kill weapon, but you also move extremely slow when carrying the flag, so it's best not to charge at someone with it.

All of the legendary achievements are doable on the Mythic Maps. I just went into Team Mythic and waited for Multi Flag on Assembly.
I just ran into the enemy team's base and grabbed the flag. Then I ran around their base for a bit waiting on a respawn. You will eventually be in close range of an enemy and the flag beatdown will come very easily.
This gametype is also good for the achievement where you need to kill 2 flag carriers.

Extremely easy to boost. Plug in a few extra controllers and find a few friends. This should be enough to make a full lobby. Keep searching an quitting if you don't find CTF. This achievement is obtainable on a Mythic map as well.

Just melee 1 person and you'll have the achievement.

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

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 the Flag Dropped achievement at the same time.