Paint the town red 20
Complete 100 Deathmatch or Duel matches played to at least 20 kills.
"You can boost with 16 bots or play these normally in mutiplayer. If you're boosting just set up a small deathmatch with 16 bots on Rising Sun and it won't take you long if you play it to 5
. If your working on other achievements like the weapon ones in this mode then it will come sooner than later."
The achievement description says to 20 kills.