If you are already using a custom firmware (CFW) like Luma3DS, you can easily switch to the 679 Build by replacing your encounter files.
Are you planning to use the for a standard playthrough or as a base for a Randomizer run ? Rutile Ruby, a challenging yet fun ROMhack : r/pokemon pokemon rutile ruby build 679 better
: The Pokémart has been revamped to discourage "item cheesing." While standard medicines are cheaper, Revives are extremely expensive, forcing players to rely on Sacred Ash (usable only out of battle). If you are already using a custom firmware
: Encounter tables are redesigned to feature Better Hordes , specifically set up to allow for painless and early EV training. Comparing the Three Main Builds : Encounter tables are redesigned to feature Better
Copy the a folder from the Encounter Type/679 Build directory of the mod files.
Paste and overwrite the existing a folder in your romfs directory.