I don't think anyone done this on Fanfiction yet but here is my personal gym. Before I start, I watched the following YouTubers done it: Birdkeeper Toby, Lockstin, Ovi, Eryizo, Ace Trainer Liam, Lumiose Trainer Zac, Pokemon Insider, Speqtor, BoltLegends, Twintendo, Super Nerd Daniel, and Mr Buddy.
Let's get started shall we? As anyone knows, there has not been an actual Dark type Gym and the type itself is one of my favorites with Mightyena and Mandibuzz as my top two Dark types in design. The gym will be the eighth you as the player need to face the based on a ninja dojo and will have three floors with invisible walls, dead ends, and bunch of Punks, Ninja Boys, and Jugglers having Dark Pokémon as well as having Ground, Steel, Bug, Poison, Fighting, Water, Grass, and Psychic depending on their Trainer Class. There will also be doors with word puzzles that will change on the playthrough and you need to find the corresponding scrolls to get the answer to the door to open it.
Unlike other eighth gym leaders, I'll have only four Pokémon and once you get to me, I'll be wearing normal clothing, sitting and drinking "special" Moomoo Milk (because why not and I am old enough to do so) and greeting you by saying that I'm no ninja but have studied them my whole live and that hardly anyone got the chance to challenge me, that you should be proud to be among the few to do so. I'll also ask for a good battle as well to get some of my boredom gone.
After that, the battle begins and I'll send out my first Pokémon- Bisharp with the Pressure ability and the moves Dark Pulse, Swords Dance, Iron Head, and Night Slash. Bisharp will be leading the charge to take out as much of your team as possible and with Pressure increasing your Pokémon's PP usage, you'll be hard pressed to take out Bisharp unless you have a few Fighting moves.
Next is Mandibuzz with Weak Armor and the moves Flatter, Swagger, Dark Pulse, and Punishment. Mandibuzz will be my high risk high reward Pokémon as it will cause your Pokémon to get confused while increasing your stats and following it up with Punishment to cause the most damage with Dark Pulse as a reserve.
My third Pokémon is Greninja for my theme of ninja and having the moves Dark Pulse, Water Shuriken, Extrasensory, and Toxic. Greninja will be similar to Bisharp minus the increasing the higher attack stat, in Greninja's case: Special Attack.
For my last Pokémon, it will be my very first starter: Kanto Raichu. Before I get to Raichu's moves, the reason I say that Raichu is my starter is because I have played Pokémon Yellow Version, the third game in the Generation 1 games and still have it as far as I know. Anyways, before I release Raichu, I'll inform you that my final Pokémon is the one that came with me on my journey since I began, so long ago. Raichu will have the Static ability and the moves Thunder, Rain Dance, Grass Knot, and Thunder Punch. Raichu will be primarily a Rain Dancer to cause lots of damage to your team, unless you got a Ground Type.
After you had beaten my Raichu, I'll tell you that I had fun battling you and lost some boredom, and I'll reward you with the Ghast Badge -it will look like the ghost from Lavender Tower minus the hands- and the TM for Dark Pulse. After giving you both, I tell you to see the unseen and always look for secrets.