@ Skara / Mark975,
I use a tool called PPRE (Project Pokémon's Rom Editor). A tutorial and download link is available
here (created by me). Please read the tutorial thoroughly.
With this tool, you can edit wild Pokémon, scripts, trainer's Pokémon, Pokémon stats, types and movesets, and a lot more! It still has some bugs though, as it is a Bèta version.
The tutorial can be found here.
(The Tutorial is slightly outdated, as the current version of PPRE is v14.0, but I'll edit the tutorial later).
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In the future, I'll not answer questions anymore like 'how did you create this hack,' you can search around for yourself.
Any more comments about the game? I'll update the game very, very soon so keep checking this thread!