You get this if you meet certain criteria in some boss fights. If you do it right, you get the achievement and a special boss weapon.
Only some bosses can give you the cheevo.
What you need to do:
Warden Garcia - chop off his right hand (confirmed)
Little Johnny - cut off both of his claws (please confirm???)
Captain Cervantes (will not give you the cheevo - confirmed)
Delver - destroy his armor on both arms in the second phase before proceeding to phase 3 (confirmed)
Delver echos - will not give you the cheevo (please confirm???)
Major General Ezra Shields - in the second phase, chop off one of his arms (confirmed)
Brother Eli revived - will not give you the cheevo (please confirm???)
Matriarch Celeste - cut off both mechanical arms on her back (confirmed)
H.A.R.O.L.D. (will not give you the cheevo - confirmed)
Goddess Helena - cut off her arm (please confirm???)
Archangel Eli - destroy all his armor before killing him (please confirm???)
If you need help with a boss, watch a video on youtube. They're really helpful.