At the start of the turn (it is B’s turn):
Player A has a 2/2 Walking Ballista on the battlefield.
Player B has a 1/1 creature on the battlefield.
Player B casts Shalai, Voice of plenty, which gives hexproof to B’s creatures.
Can player A, in response to B’s casting Shalai, then activate ballista’s remove a counter ability to deal lethal damage to B’s 1/1 creature?