To speed up the achievement progress a bit, I recommend the following:
-Play with a partner or more "if you can put extra controls better, to fill the equipment"; in the King of the Hill game mode against Bots (preferable in the lowest difficulty).
-Now the ANFITRION
for nothing in the world must LEAVE THE GAME
while the other players act killing bots or whatever the achievement requires to increase their progress. Once the progress has increased, LEAVE THE GAME
and have the host invite you again to return to the game (repeat this process each time you increase your achievement progress).
-I recommend doing it at night because there are usually few people and be agile since sometimes a different player can join.
A disadvantage is for the host because he has to stay in the game but between the members he can alternate the host.
¡REPEAT! is to accelerate a bit the progress of the achievement without having to finish the game and start a new one, I hope it helps. Good luck