This is possibly the most time-consuming achievement in any popular game to date. Averaging a kill a round, this will take you around 400 hours of gameplay to complete, not counting time spent waiting for matches to get started. What's more is, players have reported getting upwards of 13,000 kills and not having it unlock. It is possible that 'stealing' a kill (killing a downed player that you did not down) will not count towards this, or that the match must be at least 3v3 at the time of the kill, but no one knows for sure. What is known is that a lot of time and patience, and a little skill, is needed for the achievement.
For help on weapons kills, maps, and general gameplay, check out our multiplayer guide here