Just load up playthrough 2 and get a point blank kill with the shotgun and the achievement will pop.
Rehash and Resign with Horizon as Modio doesn't work.
UPDATE: Get the 4x 15 Point Exploring achievements before using this save, as it will pop the 50 point achievement for exploring all areas out of order.
I got this off Modio credit to the original maker. I thought I'd share it here as no one had posted this save yet.