At the last boss, hold Select + Start and choose attack. You'll deal a 250+ hit damage every time.
At the password screen, enter the following Japanese characters. Format is Row #, Column #.
Note: These passwords are in Roomaji. This may be difficult to understand if you don't know Japanese. These passwords will enable you to start at any chapter you like, and on different difficulty levels.
At any of the "Chapter # Push Start" screens, press Select 10 times and you'll be able to choose between any the 5 characters.