At the main menu, hold
+
then go to options. Press
and you'll have access to the stage select.
Choose the 6th stage (7th isn't available) and complete the 6th and 7th stage to finish the game.
You'll reach the bad ending this way, but you'll still unlock the trophy.
This also can be done on Easy difficulty using the cheat codes and if you sill have troubles to beat the final boss, just sign in a 2nd controller as guest before you die.