I keep dying at the second phase at the final battle. Bowser's fireballs keep killing me. I NEED HELP. CAN SOMEONE HELP ME GET AWAY FROM GIANT BOWSER AND GET THE SWITCH TO SAVE MY PRINCESS WHILE I'M DOING A SPEEDRUN? PLZ HELP ME.

practice

Jump over Bowser's fireballs by pressing 2 button. Be careful! You can also crouch if you're lucky (the fireball won't get you.)
If you are Super Mario, when you get to the platform, you'll see a propeller block. DON'T LOSE IT! YOU'LL NEED IT. 🙂

prac

I was practicing but I kept dying in the first part trying to beat little Bowser. 🙁

It's a good idea to enter that part with the smaller Bowser with a propeller cap. Simply fly over him as soon as you regain control over Mario. Stomp on that little switch and get your "princess", then as Bowser comes back, jump just before he shoots a fireball so that it hits the wall behind you. Do this until you are able to get to the next wall or until you get close to the platform area. There, use the propeller cap again to skip past most of it.

I did all of the stuff that you said. But, I can't use the propeller cap really well!

The best thing I can suggest is practice with the propeller. If you're having trouble speedrunning through Bowser's Castle with the propeller, practice playing through it casually. Speed will come the more you get used to it.
And if you're having difficulty playing through it casually, I recommend watching some videos of other players going through that part to show you the best routes. 🙂

Thank you for the information Soru. I really needed that. I will look at people practice after I will practice using the propeller casually. 😃 :D 😃 :D 😃 :D

Awesome! Well, good luck on your stream! Hope it goes well!

It did!

You're welcome! Glad I was able to help! I had a lot of trouble at that part at first as well, so I can definitely understand your pain. 🙂