The best method is to dump your own copy using a hacked 3DS with GodMode9 . This tool allows you to "build" a decrypted .3ds file directly from your installed title or cartridge.
The standard 3DS homebrew tool for decrypting your own physical or digital game cartridges into .3ds files. pokemon ultra sun decrypted
These files have had Nintendo's proprietary encryption layers stripped away. The core data is laid bare, making it readable by standard computer operating systems and software. Why You Need a Decrypted Pokémon Ultra Sun ROM The best method is to dump your own