Ok,for this achievement you need a second player and an arcade stick for boosting.In multiplayer select custom player match and arena.Select map:Vulture cage.
In advanced setting select max player:2
Auto map cycle:NO.
The second player don't move and you(the first player) need to press RT button to shoot continuously with the arcade stick.
The game start and you kill the other player.The objective is to kill continuously the other player.
In one match you will have 14 kills.
With this method you can also have the other achievement:Basic Weapon Handling level 3.
14 kills x 1000= 14000 kills
Good game and good luck.