You can stun enemies by either:
- holding X to charge your staff and then releasing when an enemy is near
- shooting an enemy with your staff whilst you have the (blue) stun ammo selected
You'll know you've stunned an enemy as they will become inactive for a short while. If you've unlocked the combat awareness abillity, they will also turn yellow.
Do this to 75 different enemies and this achievement will unlock. If you die during a fight, the stuns you've done so far will still be counted - this does not work if you select 'restart checkpoint' however.