I earned this achievement after about 16 or so hours of playing and doing easy simple challenges. In order to remove Excalibastard from the stone you need at least 2500 Badass Rank.
This weapon is found in Stantons Liver.
This video will show you how to get to the weapon.
NOTE: You can only obtain this weapon once per character, and it scales to the level to which you picked it up!!!! Best to pick it up at lvl 50
EDIT: og Yeah has confirmed that you can dashboard this achievement so that you can get the weapon later, but the achievement earlier.