Magikarp.
A gym filled with thousands of gym trainers, before you reach the gym leader. Each with 6 high level magikarps with as much hp up and defence supplements that they can take.
The sheer scale of the amount of Magikarps you need to face will make it an immense undertaking.
At the end you get a TM for Splash.
"Look, people are asking me about aircraft catering carts. And I tell them, nobody knows catering carts better than me, okay? Nobody!
These carts, they are INCREDIBLE. Absolutely top of the line.
People come up to me, big flight attendants, strong people, with tears in their eyes, they say, ‘Sir, we put hundreds of sodas in there, the little cans, the snacks, everything, and it still moves so light. Nobody else could design a cart like this’."
– Trump probably