Super Mario World/Valley of Bowser/Back Door
Your reward for making your way through the Valley Fortress? A MUCH easier pathway to get to Bowser. It's dark at first, but hit the question mark block to turn on the light above you. Right before the door to Bowser is a checkpoint. Go through the doors, where you will find Bowser in a flying contraption. Be careful of the propeller! He'll swoop back and forth across the screen; just stand in a corner to avoid him. Once he stops, he'll throw 2 Mecha Koopas over the side of his flying machine. Jump onto one of the Mecha Koopas, then throw it up so that it hits Bowser from up above. Do this again, and Bowser will fly off. Avoid the flames that drop from the sky. Peach will appear and throw you a mushroom right before Bowser reappears. Your second bout with him will be slightly more difficult, as he drops gigantic black marbles that will flatten you if you don't jump. Toss Mecha Koopas at him again, and after hitting him twice, dodge the flames as you wait for a second mushroom from Peach. When Bowser reappears, he's MAD. This time, he'll try to land his propeller on you. If you can dodge him long enough, he'll throw 2 more Mecha Koopas down. Throw them back up at him. After hitting him twice more, he'll go flying off out of control into the darkness, and Peach will appear. Congratulations! You've beaten Bowser!