Fangel! Why would you want such a thing?
For starters, the acheron is far too damaging for what it is. Adding stun can reduce the damage output to that of the other brandish lines (because status), and even open a pathway for a normal-damage 5* brandish.
Stun would only be applied on the charge, and we could add stun stars that hop out from the acheron's charge blasts. Or instead it could have a lingering mist like a griever drops for a slight uniqueness to the other brandishes.
I am actually all for that idea of balancing in that regard. And I would like a normal-damage 5* brandish as well.