Thing is, not every enemy killed drops the head immediately. They are random, so you'll have to kill a ton of them to unlock them all. Unless you are the luckiest person ever, in which case you should be playing at your nearest Casino rather than spending your time on Borderlands 2.
Either way, the below guide will help you unlock them all.
How to unlock:
- Defeat any Badass enemy
- Complete mission "Symbiosis"
- Complete mission "In Memoriam"
- Complete mission "Statuesque"
- Complete Challenge "... To Pay the Bills"
- Defeat the hidden enemy in Caustic Caverns
- Defeat the boss of mission "Best Mother's Day Ever"
- Defeat the boss of mission "Where Angels Fear to Tread"
- Defeat the first target of "The Talon of God"
- Defeat the second target of "The Talon of God"
- Defeat the target of "You. Will. Die. (Seriously)."
Each of these missions will be highlighted farther in the guide, so stay tuned for the updates.