General:
* Amazes
* Astounds
* Bewilders
* Disconcerts
* Dumbfounds
* Overwhelms
* Shocks
* Startles
* Surprises
More specific:
* Disables (if referring to a temporary incapacitation)
* Paralyzes (if referring to a complete loss of movement)
* Dazes (if referring to a confused state)
* Mesmerizes (if referring to a captivating effect)
* Hypnotizes (if referring to a trance-like state)
* Befuddles (if referring to a confusing effect)
Figurative:
* Wows
* Captivates
* Enchants
* Holds spellbound
Please provide me with the sentence where you're using "stuns" and I can offer a more precise synonym.