Kayless, the key for Ninja Gaiden 1 is to have the secret weapon jump and slash for the boss.....You can just kill the boss with only one blow !
Actually I have finished the whole Ninja Gaiden trilogy and the hardest is Ninja Gaiden 3 : The Ancient Ship of Doom - The game kinda makes you get too little time
Ryu Hayabusha - The Last Descendant of Ninja Dragon Clan