From the menu select "Continue" and then "From Logbook Entry" and enter:

¢63>¥ :tH!8 d%w!L 1G¥4M dTH6Q ¥>HLw W

as your logbook to start on the second to last level with full health, a weapon at level three, all items, and all jutsus. From here, keep heading up and go through the doors to the castle. You'll meet the king who will tell you to go find the secret passage. Exit, go down one screen, go left until you see three statues. Beat the middle one until it breaks. Enter and you're on the second to last side-scrolling mission stocked to the gills. (Because there's no 'heart' nor 'triangle' character on my keyboard, I've used ¢ to mean 'heart' and ¥ to mean 'triangle'. Make the appropriate substitutions when entering the code.)