I'm just so proud that after 2hours of trying to find the solution for the puzzle this Gym is that I could beat it, that I wanted/needed to share it.
I tested extensively, and it seemed completely impossible. I couldn't set sweepers, their pokes were too damn fast and hit too damn hard, I barely had options to change fields, as I had to Focus Sash only to set Electric Terrain, up until I found a way into tricking the AI to make a switch that gave me the upper hand.
In my opinion, being very unexperienced in competitive pokemon, but having learned these past 2 years a lot by just playing Reborn/Rejuv on their Hardest difficulties; I'd say that this was the hardest fight I've had so far.
I hope it doesn't get worse from here onwards, or I doubt I'll make it to the end. https://drive.google.com/file/d/1HLd4NdwUV8ntcae_htT8L6C44lHLa78v/view
PS. Thanks for the all the community tools (Monotype guides with pokes per gym, Items/Pokes locations, etc).