Evie (ChickasaurusGL) 🌺 | Glitchfest where Magikarp solos Elite Four and Champion using moves+no items in battle (Green JP) @ChickasaurusGL | Uploaded March 2024 | Updated October 2024, 4 hours ago.
There is a lot to explain specifically sorry, however, I heavily rely on the Select glitches https://glitchcity.wiki/wiki/Select_glitch

Specifically,

https://glitchcity.wiki/wiki/Dokokashira_door_glitch
https://glitchcity.wiki/wiki/Skip_to_Level_100_Select_glitch

Select glitches for moves beyond slot 4 to teach Magikarp moves it normally can't learn. This does not always match the move it will acquire in battle as opposed to outside of battle (similar to a temporary Transform move).

Select glitch expanded inventory and
https://glitchcity.wiki/wiki/Walk_through_walls_glitch_(Select_glitch_method) (add close bracket if link doesn't work)

Another Select glitch walk through walls, adapted from

https://glitchcity.wiki/wiki/Walk_through_walls_glitch_(international_Select_glitch_method)

I enter battle and set the swap position to 62 and perform the Select glitch; overwriting CD38 and allowing me to walk through walls. I then use a partial escape glitch item (はやぶさバッヂ (Hayabusa Badge or FalconBadge) 0x65) so I partially escape battle with the corrupted CD38 value and walk through walls, as leaving the battle under normal circumstances would reset it.