Aliens vs. Predator

Start Marine campaign. Or load first chapter.

Pistol gun is rubbish so it's good to get this one out of the way quickly.

You will get a pistol right at the start of the campaign.
Keep plodding through the story until you get to a point where you see a barricade and some guy looks desperately at you through the barricade and gets dragged back in which you and some chick fight with a turret and the aliens (xenomorphs) are scurrying through the gaps in the barricade.

You will be told to go somewhere else but just keep where you are as the aliens will just keep spawning and yes you will have unlimited ammo with your pistol even though it looks like you don't.

Get 30 kills and the achievement pops up instantly.

At the start of the marine campaign you get a hand gun simply kill 30 xenomorphs or do a solo survivor on any of the maps.

If like me, you didn't do this during the campaign and can't be bothered to start it again, then just start up a game of survivor. I did this on C-block together with the achivements "using 30 stims" and the "5 killstreak with the smartgun".

Just run backwards and take sharp turns once in a while to avoid getting hit, while you spray the aliens with secondary fire. The burst is way better than the useless primary fire.

Very easy way to do this is to set up a lobby with you and a friend, Set up a player match in Deathmatch and set the maximum number of players to 2 and private slots to 2 on any map, be sure to go into Match Settings and set the amount of kills to 50 and the round time to maximum allowed, then just start the game with you and your boosting partner both as Marines, find each other and kill with the Pistol, repeat this for each player til you all have the achievement, this method also works for the Headshots with a Scoped weapon achievement.

Simple :-)

Enjoy

This is easily obtainable during the first two missions of the Marine Campaign.

When facing one or two xenos at a time, use the pistol. There are certain points in the second mission where you will be overrun by aliens (meeting your general, during the fight with the queen), so your shotgun or machine gun will be preferrable.

Also, whenever you encounter a facehugger egg, always use the pistol. 2 bullets and the facehugger dies, those along with a few fully grown xenos will net you this achievement easily.