It’s easy, regardless of game difficulty, to score a Ninja 5 rating on each mission in the game. If you go undetected, uninjured, and use no items, you’ll get 300 points right off the bat towards your rating (100 points for each). Since you need 900 points total to reach Ninja 5, you’re only 600 points away if you get the initial 300 points. Stealth kills are worth 120 points each, so killing five people by stealth means will earn you the points you need, though stealth killing a boss enemy is worth 300 points. Regardless, with these numbers in mind, scoring Ninja 5 on every mission is easy.