User blog comment:HarutoR/Shakogairu, Water Academy No. 9 effect/@comment-32043044-20190614060718

A Ninja Strike Ability states: "(Whenever one of your opponent's creatures attacks or blocks, if you have # or more cards in your mana zone and if during that attack you didn't use a "Ninja Strike" ability, you may summon this Shinobi for no cost. At the end of that turn, put this Shinobi on the bottom of your deck.)"

When the attack hits the player (or creature), it is regarded as after the attack.

This means that the player who owns Shakogairu reaches their Win condition before the attacking player would, and therefore is victorious.