Treason of Tzeentch vs. Multiple Opponents
I've got a 4-player free-for-all match against some friends this weekend, and I have a question about Magnus' 'Treason of Tzeentch' ability. Here's the current rule:
"Treason of Tzeentch (Psychic):
At the start of your opponent’s Shooting phase, select one enemy unit within 24" of this Psyker. Until the end of the phase, ranged weapons equipped by models in that unit have the [HAZARDOUS] ability."
Would I be able to use this during each opponent's individual shooting phases or only once per battle round?
Obviously this is up for interpretation, and while I would certainly like to use it during all of their individual shooting phases, they will probably read it differently, so I'd like some rules to back up my argument.
Thanks!
*Edited for reading clarity.*