Ninja Gaiden Sigma Plus

Go to your equipment section in the pause menu and select Ninpo > then select Inazuma. You find this in chapter 15, after defeating the ice boss. Its in a secret box in the next room. You must upgrade to level 3 for it to count.

You will get this in time. The reason it takes so long is because you're limited to a few Ninpo usage. Trophy will unlock after Successfully using your 100th Ninpo.

*Note* When you use Ninpo, there will be a mini game with your touch screen. If you complete it, your Ninpo will become stronger. This is confusing...I'm not 100% sure that you must complete the mini touch screen game in order for it to count towards trophy. I advise you to try to complete all mini touch screen games. To use Ninpo, press + when your Ninpo is available.

Also there is a way to get this trophy very quickly. If you find one of those Team Ninja logos, it will refill your Ninpo. Use all your Ninpos and refill again. Another method is to play Hero Mode. When you're health is low you will activate Hero Mode and have unlimited Ninpos. These are best methods to achieve trophy since you'll have unlimited Ninpos from refilling.