QUAKE 4

This is pretty easy to get on the (I think) fourth level. You should obtain the shotgun partway through the third level, so you can start using it for this purpose at the beginning of the 4th level. To help, turn off "Auto switch weapons on pickup" so you don't accidentally fire another weapon when you pick it up.

Also, I found that especially on easier difficulties and on the earlier levels (1 through 6 or so) , you can hang back and take cover while your squad mates do alot of the work. This helps you conserve ammo which can be important when you can only use one weapon all level!

After you finish with the last tower (the Data Processing Security tower) go to lift just right before the level exit and save the game. Keep this save game alive so you can farm this file.

Pick a weapon in your inventory and exit the level. Upon loading the next map, do not change your weapon and proceed to kill the big monkey strogg enemy before you approach the tram and exit this map.

Simply reload the save game from the Data Processing Security tower and repeat the tram map with each weapon and you'll bag all the achievements quickly.

In options under game settings just turn off auto weapon switch so that you can pick up a different weapon and not screw yourself out that weapon achievement. -Conker1182

The other solutions are great but if you wanted to do the achievement without saving and farming off of that then you can get the achievement on any of the tank levels. Just make sure that you have the shotgun or any other weapon for the achievements equipped before you get in and fire it just to make sure you get the achievement.

I assume this method works as the guns on the tank (Machine gun and rocket canon) don't count as they are part of the vehicle not your inventory.

In my case i didn't need to kill anyone beforehand but it is probably a good idea to get a kill before entering one of the vehicles.

The video shows the section that i obtained the achievement which is from the beginning of the 8th section of the game. There is also an entertaining commentary but the video is just to show the part of the story that is the quickest for this achievement.