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 (verified)
Little Johnny - cut off both of his claws (please verify???)
Captain Cervantes (will not give you the cheevo - verified)
Delver - destroy his armor on both arms in the second phase before proceeding to phase 3 (verified)
Major General Ezra Shields - in the second phase, chop off one of his arms (please verify???)
If you need help with a boss, watch a video on youtube. They're really helpful.