Dearest Of All My Friends Achievement
Dearest Of All My Friends10
Kill Someone On Your Friends List
This can be done in any multiplayer playlist. All you have to do is kill someone from your friends list once and this achievement will unlock. Deathmatch would be the easiest to get this on, just join a game and then get a friend to join in with you then kill them.
You could also invite players in the Deathmatch to be your friend and delete them after the match.