To get this Achievement, go into the Legendary playlist called DLC Big Team. Wait until you get a match on Avalanche with Capture the Flag. An easy way to do this is to drive into the Giant Man Cannon at the enemy base and fly into their base with a vehicle. Then pick up the flag and wait for someone to come running. Melee them with the Flag and you will get the Achievement. (Flag kills are instant death for the enemies, unless they have Overshields.) A better variation on this is to move down the ramp once on the top level of their base with the flag, and crouch near the opening where the ramp changes direction, as people will come running to defend the flag, and in this position you can beat them down as soon as they come round the corner, lessening the gap for them to shoot at you. In this way you will also avoid being sniped which is a risk when camping at the top of their base.
An easier but less common way on the map Avalanche is to grab the flag then head down the ramp and take the portal, which will teleport you to where the turrets are. If you are lucky then a helpless enemy will be using the turret, allowing you to assassinate them with the flag. Crouching may help to avoid showing up on their radar.
Luckily, there are other ways to do this achievements, because DLC Big Team isn't the only playlist that includes CTF and Legendary Maps. For example, Team Doubles is fine to do this. There is also a DoubleEXP weekend entitled Team Flag, which requires Heroic and Legendary Map Packs. If you get the chance, wait for a few matches to get Legendary Map and wait for a good moment to melee someone with the flag..
- This achievement could have been named after a level in Marathon, "Defend THIS"!
- You will get this Achievement if you stick a player on the opposing team and pick up the enemy flag before the grenade detonates, proving that you don't necessarily have to kill them with a flag melee, but just get a kill while holding the flag, although this is hard to do without using the flag's insta-kill melee.
- You can also get this achievement while playing a CTF match on any of the Mythic maps. It is unknown if this is a glitch or if Bungie intended to make this achievement unlockable via the Mythic Map Pack.