That could have been part of the plan in the first place, to make SA perform the spell over and over again, but she controlled when the attack would happen and waited for him to be exhausted, even nulling his pain to keep performing it until he was out of juice, which we've seen that Unicorns and even alicorns can "run out of power" and need to recharge. (Twilight ran out of magic during the time travel and was helpless in the nightmare moon timeline for quite some time).