Bowser in the Fire Sea

N64
We can't get any more Stars until we beat Bowser for a second time. The entrance is in the same room as Dire, Dire Docks, in the raised hole in the floor. The level is covered in lava, which is why I died 9 times trying to get to Bowser.
To start out, we ride a metal grate across the lava. It will go under a platform with a 1-up and will dip into the lava afterwards. Jump to the tilting square platforms to the left for safety. It will reach the end of its track, where we jump off onto a stone platform with Bullies and a 1-up, as well as blue, triangular platforms that dip into the lava.(Many of the platforms here bob up and down, which will force you to wait until a platform or item comes into reach.) There is a metal grate ramp to the left. It ends in a square ring. Over the hole in the ring is the first Red Coin. Back on the stone platform with the Bullies, we continue forward where the path ends with a pole. We climb up the pole and jump onto a tilting stone platform, where we find the second Red Coin.

There is another pole near the tilting stone platforms (slightly closer to where we started), which leads to a small cage. At the top of the pole, a 1-up will appear. The third Red Coin is in a corner on the opposite side of the elevator. If we run onto the elevator to trip it, then run off, we can access the 4th Red Coin by jumping down to where the Bullies are. After getting the 4th Red Coin, we take the elevator to the top, where there is more lava and stones that shoot out flames. We cross the lava using the moving stone platform. There is a blue slope at the end of the short ride. If we run up the slope, but not down, we can access a grate ramp for the 5th Red Coin near a Bully. Past the slope is a grate we can hang onto, with coins underneath, or just walk over (I recommend walking over). Either way, we'll use it to cross the lava. After crossing some moving squares, we'll go up some more ramps, getting a 1-up from the item block that periodically floats out of reach. After the third ramp, the 6th Red Coin floats in and out of reach as well. It's past the piece of the platform with the hole. At the top of the ramp is a different ramp, that will take you to the top of the level. It will float in and out of reach, so don't be surprised if you don't see anything there at first.

At the top of that ramp are fire-shooting stones and blue, triangular platforms bobbing in the lava. The 7th Red Coin is right in front of the second fire-shooting stone. Next are three poles bobbing in the lava. Above the third is the 8th Red Coin. The Star will appear on top of the high platform near where you found the 7th Red Coin. To get to it, I backflipped to grab onto the thin ledge, and walked to the Star. You can also wall-jump between the two platforms.
Star Get #72, 51 Coins, 14 Lives

Now, time to take on Bowser! Past the third pole are some fall-away yellow platforms leading to a green funnel. That funnel leads to the battle arena. We land on a large red disc. Bowser appears, saying "Pestering me again, are you, Mario? Can't you see that I'm having a merry little time, making mischief with my minions? Now, return those Stars! My troops in the walls need them! Bwahaha!" This time, when Bowser jumps, the disc will tilt, possibly leading Mario to slide off the edge into the lava. It is possible to recover from the lava, but it's not easy. He can also warp, making it difficult to catch him. He also spins around more quickly. I died twice battling him. All we have to do is go to the edge of the disc, preferably near a bomb, wait for him to chase you, dodge, and grab his tail, then throw him into a bomb once.

After landing, Bowser says "Ooowaah! Can it be that I've lost??? The power of the Stars has failed me... this time. Consider this a draw. Next time, I'll be in perfect condition. now, if you want to see your precious princess, come to the top of the tower. I'll be waiting! Gwa ha ha ha!" He dissolves into the key. We get the key, with a "Here we go! Yahoo!"
0 Coins, 12 Lives

Nintendo DS
We find the entrance to the level in the same place as the N64 version. Not much has changed. The Red Coins are in the same place. I died three times trying to reach Bowser.
Star Get #77, 46 Coins, 12 Lives

There is a Switch Star in this level as well. The Star Switch is just before the platform with the Red Coin Star, near the item block with a 1-up. The Star appears at the top of the last ramp, near the Bob-omb. To get it, I long-jumped to the Red Coin Star platform, then long-jumped down over part of the lava, (while the platforms were bobbing up) and long-jumped and ran to the Star. You do not want to deal with poles; they will slow you down.
Star Get #78, 13 Coins, 17 Lives

Now, we'll beat Bowser! The entrance to the battle arena is the same. Bowser's speech doesn't change much, although he does not call a character by name, and he says he stole the Stars first. His laugh is a Gwa ha ah! I beat him my first time, using the same strategy. His defeat speech is the same. He turns into smoke and the key. It is another swirled key.
15 Coins, 18 Lives

Now we can get some new Stars and advance to the third floor! We're almost done!
The next Star is Pole-Jumping for Red Coins.