This challenge is not killing the enemy flag carrier. If your team drops the enemy flag and the enemy is attempting to return it by standing close to the flag (within the highlighted area), kill the enemy for the challenge.
Lol, this aint even one of the hard ones, count yourself lucky!
To be fair though, I had this one - I found myself letting my teammates die with the flag so the enemy could start returning it, for me to then swoop in and get the kills. Felt dirty doing that to my teammates, but hey, 343 were forcing my hand.
It’s still an awful challenge though. Not that easy to have decent teammates who pull the flag, and then you being in a position to kill people returning it.