(CAN NOT BE DONE IN PRIVATE MATCH)
This will come naturally. All you need is a total of 100 kills online. Play either Team Deathmatch, Deathmatch and/or Gang Wars, really whatever game mode you feel most comfortable & confident with. Achievement unlocks as soon as you hit 100 kills.
*Playing Gang War would be the quickest way to get 100 kills. Since there are 5 different rounds per game.