Check out Latios212's piano cover of this theme! Their sheet music helped me to transcribe the MIDI:    • Pokémon Legends: Arceus - Encounter i...  

MIDI: https://www.mediafire.com/file/lfvan3...

What I use for the music:
Anvil Studio (MIDI composing/sequencing and looping)
Polyphone (Soundfont editing software)
Sappy 2006 + Mid2Agb (Adding MIDI to GBA ROM)
mGBA (Playing the song in-game and recording the audio)
Audacity (For fading)