If you were to use a save editor, you'd:

: Use cheats to make all wild Pokémon a specific one or use a device like the PKHealer and encounter modifier.

If you’re looking to skip the grind and jump straight into high-level battling or breeding, a is the perfect solution. Why Use an "All Pokémon" Save File?

: You will need a hardware tool like an R4 card or a GBxCart RW to dump and inject save data between your PC and the cartridge. How to Start Your Own New Save