Smashboards

You can use the 'Select by Color' tool to select the magenta. Then you can grow the selection by 1px, feather it (amount varies depending on your resolution), and then delete the selection. Starting with a high resolution image (like 1080p) will improve the quality of the finished product.

Or, you could skip growing the selection by 1px (which will result in the pink border you mentioned), and later add a 1 or 2px black border on top of that area instead. That'll give you more defined edges. Some of the game's vanilla CSPs have such a border too, like Falco, Ganon, Link, etc. Either way, the effects of the edge will be reduced when you downscale the image.

Alternatively, if you use a black background instead of magenta, you can try the script I have here for removing backgrounds, which might give you better results: https://smashboards.com/threads/csp-collection-updated-frequently.443399/#post-21589504

Awesome. That first method is working much better. What's the process for adding a black border? Also, I'm getting this weird issue rn where I cut my selection, but it leaves part of it. Like I select falco and cut it, but it leaves sections of falco's boots. Any idea how to fix that?