If "Sasuke Samurai" attacks a face-down monster, apply its effect after entering the Damage Step, but before flipping up the face-down monster. The monster is not flipped. Flip Effects are not activated. Monsters like "Kisetai" and "Giant Axe Mummy" do not activate their effects.
If "Sasuke Samurai" attacks a face-down monster, there is no damage calculation. "Sasuke Samurai" destroys the monster before damage calculation is reached.
If "Sasuke Samurai" destroys a face-down monster using its effect, the face-down monster is NOT "destroyed as a result of battle", it is destroyed by "Sasuke Samurai"’s effect. So "Mystic Tomato", "Helpoemer", etc. will not activate their effects. However, monsters like "Sangan" will still activate their effects.
If "Sasuke Samurai" attacks a face-down monster, and "Ceasefire" is activated during the Battle Step to flip it face-up, "Sasuke Samurai" does not destroy it with its effect and damage calculation is applied normally. The monster must be face-down at the beginning of the Damage Step in order for "Sasuke Samurai" to activate its effect.