Grand Theft Auto IV

There is a much easier technique than the previous one posted.
Host a ranked deathmatch and change the weapons type to pistol only, and put a long time limit on the game.

the easiest way to do this easier than those 2 is host a deathmatch . 2. change the map to the prison as it is the smallest map 3. weapon type to pistols only 4. unleash hell with a pistol and once you killed 20 in one match once done the achievement shall pop up and then you can either see out the rest of the rest of the match or quit it.

The best way I found was simply host a "Ranked Deathmatch" put it on "standard weapons" and everyone gets a pistol and put the timer on "30 minutes". I got it within the first 10 minutes and rest of the time I just messed around. If you do this you have a less of a chance of getting kicked from being the host. I also put respawns "Near" and the time to respawn "1 second" at times you have at least 10 players shooting at each other or trying to run away in cars. The game was kinda hectic but still fun at the same time

(Please timely freedback on this solution or any other solution I've done.)

I just got the achievement today and since I don't see the boosting-strategy here that we used I will provide it now. It all comes down to get a lobby of your own to get the boosting done. The strategy is the same like for the races:

1.) If the lobby is too full just let the game start and immediately search for a new one (with the cell phone) as soon as the game starts. All of the other players are still in the game and you get a fresh lobby - maybe with other non-boosters though. So you might need to do a new search a few times after a game started.

2.) When your boosting group has the majority of people in the lobby you can kick the other players out of it. Not very nice, but life ain't always nice :)

3.) If you have your own lobby don't forget to set the weapons to pistols! :) Put the game to 30 minutes or so to be sure, the achievement will pop as soon as you have enough kills, so no need to sit the game out completely.

We did this with just two people and got it in a few minutes!

This can be done with one other person. From the singleplayer game, pull up your phone, enter the Menu and select Multiplayer.

You’re going to want your preferences to look like this:

Auto-Aim
Disallow

Police
Off

Friendly Fire
On

Setting it this way allows for the likelihood of crossing randoms to be a minimum. Make sure both players going for the achievement have it set this way.

Now start up a Ranked Deathmatch. If you don’t match up right away, keep searching for a new game (Back button) until you do.

Set the game settings to look like this:

Location - Prison
Duration - 10 mins
Weapons - Pistols
Respawn Distance - Near
Respawn Time - 1 second

Start up the match and after you get your 20th kill with the pistol, the achievement will pop.