When covering giant-monsters from ninjitsu films, the first that comes to mind is The Magic Serpent, although its original title was Grand Duel in Magic. The giant monsters were not only a dragon but also a giant toad, which although sounding ludicrous at first, is actually traditional from Japanese folklore. In fact, the toad can be even found in Kabuki.
Portraying the main hero is actor Matsukata Hiroki, although during that time, his Father, actor Konoe Jushiro, was much more popular, acting as the historical Swordsman, Yagyu Jubei, appearing repeatedly in sequels of Yagyu Secret Scroll. Later on, though, Matsukata took over his Father's role in a television series on Jubei. That is the series I most want to get, so if anyone has information, it would be appreciated.