It is extremely unlikely that you will ever find anyone playing this game online anymore, so find yourself a boosting partner (this can be done with just 2 people) and host a Ranked Team Deathmatch and set the kill limit to 5. When the game starts you will be prompted to hold down Y to choose Graves or Lang. While holding Y choose the character on the right and don't change for the remainder of the game. Just kill your opponent 5 times and this achievement is yours.