The saving grace on Ninja Gaiden is, although it sends you back after losing to one of the 3 final boss forms, you never have to re-battle any of them that you kill. So you can essentially go into the boss room, spam that diagonal fire attack, die on the next form, make your way back to the boss room, rinse and repeat. There's no friggin' way I'd ever have been able to beat Ninja Gaiden if the game made you kill all 3 boss forms in a row.