“Ninja Grandmaster Sasuke”’s effect activates at the beginning of the Damage Step, before the monster is flipped face-up. So if “Ninja Grandmaster Sasuke” attacks a face-down Defense Position monster, “Ninja Grandmaster Sasuke” will not destroy it with his effect because it is not flipped face-up until after “Ninja Grandmaster Sasuke”’s effect would activate.
A face-up Defense Position “Spirit Reaper” will be destroyed by the effect of “Ninja Grandmaster Sasuke”. “Ninja Grandmaster Sasuke”’s effect does not target. So if “Ninja Grandmaster Sasuke” attacks “Spirit Reaper”, “Spirit Reaper” is destroyed by “Ninja Grandmaster Sasuke”’s effect, not by “Spirit Reaper”’s effect.
If “Ninja Grandmaster Sasuke” attacks a face-up Defense Position “D.D. Warrior Lady”, “D.D. Warrior Lady” is destroyed by the effect of “Ninja Grandmaster Sasuke” and “D.D. Warrior Lady”’s effect is not activated.
If “Ninja Grandmaster Sasuke” attacks a face-up Defense Position monster, and you flip it face-down during the Battle Step with “Book of Moon”, the monster is face-down at the start of the Damage Step so “Ninja Grandmaster Sasuke” will not destroy it with his effect.
If “Ninja Grandmaster Sasuke” attacks a face-down Defense Position monster, and “Ceasefire” is activated during the Battle Step, the monster is face-up at the start of the Damage Step so it will be destroyed by “Ninja Grandmaster Sasuke”’s effect.