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. Even as a level 1 you will have acess to 5 default loadouts, one of which has Tee Hee as a playable character. (He is the african american gentleman with the red suit and glasses if your not sure)
Start the match and melee a minimum of 3 enemies (more dosent hurt) You have to have the most melee kills at the end of the match for this to pop NOT the most overall kills. If your having trouble try camping around corners.
Also, i found when we boosted this that his melee attack isnt always a 1 hit kill. You may have to hit them twice or put a round or two into your target with your pistol.