Super Mario 64/Bowser in the Fire Sea

To enter the Fire Sea, you must have 31 stars and have beaten Bowser in the Dark World to get the key to the basement. When you go through the Star Door in the basement for the first time, the entrance to Dire, Dire Docks will be blocking the entrance to the Fire Sea. Beat the first level of Dire, Dire Docks to make it uncover the entrance to the Fire Sea.

Normal route (no red coins)
Mario will drop into the Fire Sea, but there will be no message from Bowser this time. Begin by running up to the platform in front of you and stay on it as it moves through the lava. Avoid the wall in your way, then jump onto the two platforms to the left side to you won't get burned when the moving platform goes under the lava. Jump back onto the platform and ride it to the end. Jump onto the blue platforms above the lava and jump to the next gray one (be careful on the blue platforms, as the lava moves on and off them slowly). Avoid the bully and the Goombas and run to the end where a moving pole is sticking out of the lava. Avoid the electric sphere and jump onto the pole, then jump onto the platform above. Turn around and jump onto the tilting gray platforms, and then jump onto the pole at the end of the second one. Climb up the pole and jump off, and then walk over to the gray elevator to get up to the second level of the stage.

Once on the second level, run straight and long jump over the gap (be careful of the machine spitting fire in between). Run over the blue triangle, and then position yourself under the platform with the coins under it, and then jump and hold onto the bottom of it with A. Still holding A, climb over to the other side. Cross the moving platforms and avoid the two bullies ahead. Walk up the series of moving ramps and platforms to get to another gray platform. Run up the platform and jump over the series of blue platforms in the lava (avoid the fire-spitting things and the moving lava). Jump onto the platform with the spinning heart and then jump onto the moving pole. Jump up to the next platform and keep jumping onto moving poles and platforms until you reach the gray platform with the bridge in front of it. Run across the bridge quickly (it falls apart as you do) and jump into the large pipe with an image of a star in it to reach Bowser.

Red Coin route
Under construction

Defeating Boswer
To defeat Bowser, do the same thing as you did with the last one; grab his tail and throw him into a bomb. Be careful this time though, as Bowser tends to jump and pound the platform, which causes it to tilt. Run to the other side to avoid falling off. Also, Bowser can teleport around the platform, so keep an eye out for that.