First off you NEED the Mechromancer as a playable character or this won't work for you.
Make sure you play on True Vault Hunter Mode.
All you have to do is shoot an enemy point blank with a shotgun and the achievment should pop.
I made this save because I searched the internet high and low for a proper PC version of a WORKING save file that isn't an XBOX version which required conversion back to the standard format using programs that never did the job right.
Not sure if you need the profile.bin file but it's there just in case.
Enjoy ;)
-rEd2k (Redkilla)