Load up the level LOST TUNNELS. Follow your comrade, Bourbon until he will tell you about a tripwire. After you jump said wire, Bourbon will tell you about some bandits that have barricaded themselves and if you can take them out. This is a good spot to boost this achievement.
Now just start shooting the bandits in the first big room and after everyone is dead make sure to reload checkpoint. Reloading checkpoint does not reset your progress towards the achievement.
If you start the level and you don't have a pistol, you can instead load up the level RIGA, which is the level right before LOST TUNNELS. Buy a pistol form the vendor located under the bar where you start.