Made a full playthrough of this game.
I am still struggle beating Ninja gaiden tbh but i have some experience with Ninja Gaiden 3 which is easier because i played the japanese version of ninja gaiden 3(The english version has increased difficulty which i beat after i am done with japanese one).
And about the first ninja gaiden, I ever beat it but with save states since i have issue with the enemy respawn infinitely.
What i am impressed the most is how you redesign the last boss. While it's still hard, The demon head who originally just ran and unavoidable now bring a call from the earlier boss you just beaten in this game.
Another thing i am impressed is all boss are now have invicibility time when get hit. This give me feel when playing Ninja Gaiden on Master System or Mega Man
Overall it's 9.5/10 for me.