A good place to get this is on the location shown on the guide for:
This is Level 21 MEETIN. Take out the first three guys and then go around the next turn. Quickly take out the guy with the shotgun and grab it, or stun him and kill him with the shotgun for an extra shotgun kill. The door will open now. There will be one guy straight ahead and two more will run in after some time. Killing all of them with one shot will net you the conserve ammo achievement. Kill them, and then kill the last person in the next room. In this room, there's another shotgun on the left side. Grab this, as you may be running out of ammo now. More guys will come to attack you. Kill them all, carefully using cover and timing to avoid their rapid shotgun and rifle bullets. One of these guys carries yet another shotgun. Grab this if you need it and continue killing the attackers. This nets you shotgun kills per run. If you work on this after completing the story, you won't have that many kills left to attain. Continue until you reach 100 shotgun kills to unlock this achievement.