I found the Railgun blueprint under the first room turning puzzle in the VDNH Exhibition complex. Once you own the blueprint head to a crafting station and get the Railgun crafted. Unlike other weapons the Railgun will need 1x Neuromodule which only drops from boss fights. You'll need to kill Hedgie or Belyash to aquire one. With the Railgun in hand head out and lure 3-4 enemies together and run backwards until they start to line up. Fire the Railgun through them all and the achievement should pop. I did this with mutants due to where I was. But you could quite easily do this with any of the other mob types.
If anyone has spotted the Railgun blueprint elsewhere please let me know in the comments:Blueprint Locations:
- Under the first room turning puzzle in the VDNH Exhibition complex (Lordacris).
- On the dock where you first activated the Hawk to get into Hedgie's arena (Nesmaster).