My opponent removes all counters from Walking Ballista and says 'hit you for 10'. I then cast Selfless Squire. My opponent then says 'no, I do it one at a time'. Is he allowed to do that?
When a player does something like activate Walking Ballista 10 times in a row, they’re not activating, holding priority, activating, etc. There is no reason to ever play this way with the exception of a few effects such as split second spells. Instead, “activate Walking Ballista 10 times” is generally understood to be a proposal of a shortcut to activate the ability, let the activation resolve, and then repeat that loop 10 times. You can respond at any point in this sequence, but unless your opponent specifically says that they’re holding priority and activating all 10 times without passing priority, then you’ll only ever be able to catch one activation with Selfless Squire.
