Flaming Ninja Anniversary
To unlock this achievement you must kill an opponent with this current achievement:
It must be done in matchmaking so either you can get lucky and kill someone with this achievement, or you can play rumble with a friend and kill them.
I got the achievement in Ranked Team Halo 2: Anniversary playlist on the map Zenith killing a random who had the Ninja Redux achievement.
According to gomuhimo RSC
he shot his guest account in Halo 2A and obtained this achievement when he had the Ninja Redux achievement. Also confirmed by others so you can shoot your guest account if you have the ninja redux achievement yourself, you will unlock the achievement!
Note: Todays patch update (12/22/14) they fixed it where if you kill anyone with this achievement you will definitely unlock the achievement. Meaning if you boost the assassinations with a friend online and then kill each other you will obtain the achievement.