Dropping Acid Achievement
Dissolve 50 enemies with acid.
To do this, you must have the Acid Bath weapon part. This part is found in Chapter 11 in the optional quest "Armory". You must get to the end and open the container with the rewards. Attach this to your weapon at a workbench. When you shoot enemies, they will start burning from acid. You can get this over time throughout your playthrough. If you want to get it all done at once, you can play the Co-Op mission in Chapter 14 while playing as Carver. At the final nightmare you will get to the end where you have to destroy the symbol in front of you. Instead, stay next to it and shoot the frail feeder shadows. Move forward to respawn some. They will sometimes stop running at you when in certain spots around the symbol making it pretty easy to maintain your kills. Just be warned, that in Co-Op play, your stats will often reset if you die or quit out.