This achievement has to be done in the classic conflict game mode in a public match. You will need to be rank 35 in multiplayer OR from the main menu go to extras and select cheat codes. Enter the code pa1ntth3townr3d and the mode will unlock.
You will need to be level 8 in multiplayer so you can customize your loadouts to be able to select all the playable characters. You MUST play the match start to finish to count. Playable characters are: