It Hammer Shot is activated and Strike Ninja is indeed the Highest ATK monster on the field. Strike Ninja can remove from play 2 Dark Monsters which is step 1, and then Hammer Shot effect is activated so it'll destroy the 2nd highest ATK monster on the field.
So Hammer Shot will destroy something unless there's no other face-up monster on the field other then Strike Ninja that removed itself from play.
So Hammer Shot will destroy something unless there's no other face-up monster on the field other then Strike Ninja that removed itself from play.