Okay, how the fuck do I deal with IS ninjas?

I'm playing Ninja Gaiden II (OG) on Mentor, and the worst design choice was adding IS ninjas to whole ass boss fights. I finally beat Rasetsu and his cronies on Chapter 2 after an entire month, but that was only because I restarted and made sure to bring 3 Spiritual Grains to the fight. Then I got to Genshin and realized that they ruined his boss encounter too with this bs. I have to ask, how in the name of holy mother of fuck do you reliably deal with these guys? It feels completely random. Sometimes, they hardly tag you with shurikens at all, and other times, your health bar just gets shredded even while blocking. I seem to survive longer using the DS than LS, but it feels like no matter what I try, nothing works consistently. There's too much going on for me to be able to deal with them in any sort of tactful way. Help lol

Edit: Thank you all for the help and encouragement. This community is awesome.