For a run I'm doing I'm going through 'Donut Secret 1' and the professional speedrunners do this trick to kill enemies while still holding on to a shell:

I don't know how to do it, I just keep throwing the shell away. I'm playing on a gameboy advance, but knowing how to do in on SNES would still be helpful. I hope that the GBA port isn't lacking the inputs or something

Game-specific questions like that should be asked in the forums/discord of the game, instead of the general forums, where you can get better help frm the community. Just search for the game in the top text box.

https://www.speedrun.com/smw/forum

@Oreo321 Actually, it seems this user is playing Super Mario World's GBA port, which is a separate game titled Super Mario Advance 2: Super Mario World. The appropriate forum would be here.

On SNES, while holding the Y button you press the X button, or hold the X button and press the Y button. On GBA I guess you can try holding the B button and press the R button or whatever the spin jump button is?