To obtain this achievement you must kill someone else who has this achievement:
Yesterdays patch (12/15/14) fixed this achievement and it can now do it in custom matches with a friend. If you have no friends with the Legendary anniversary achievement they can be found in matchmaking.
If you have a friend with it willing to help you can get this knocked out of the way, but otherwise don't waste time trying to find someone, work on the other matchmaking required achievements and you could easily get this one by chance.
If you are really having a hard time getting this achievement and can't seem to kill someone with it just add my XBL account and send me a message and ill work at helping you get it.