So I had a hard time getting this achievement at first but I found a way that is kinda fun.

-Requires BFG and some unfinished bandit dens
-You can also use the rocket launcher but it takes some better timing and it might kill less

Go to Drop Dead Canyon, take the elevator up, and kill nothing. Just sprint and dash through the base until you get to the upper area. (Where the zip lines to the lower area are and the big boi boss comes out). Charge up the BFG aiming it at the goon squad on the lower side and right as it fires jump onto any of the ziplines. Get three or so kills and reload your autosave right before you enter the base and repeat. if you'd rather bring a couple rounds and continue to kill as many people as you can by luring them into the lower area and firing into the other part of the base with more goons.

You can also go to Stiltown, run to the top of the tower in the back where the zipline is and fire into the open areas with goons.

Sorry for low quality, just wanted to make a quick guide because I struggled with the achievement and its at 1.1% of all players.

Good lucks