SwampySwampert take
Mystery the _______, where ________ is the first Pokemon you find in the Route 22 Hidden Grotto. (If you accidentally kill it, you have to keep resetting the grotto until you encounter the same mon you killed the first time.)
If ________ is Amoonguss, Mystery must switch out anytime it takes a hit. You may also never use healing items on it. Defeat Marlon, where Amoonguss is the only Pokemon that uses a move on Marlon's Pokemon (I can't say solo since you have to be switching a lot but it's the idea). Lead with it against Kyurem-B.
If ________ is Pelipper, it must learn Rain Dance and Roost right away. If the weather is not Rain, it must use Rain Dance, and if it is below 50% health, it must use Roost (and cannot use healing items below 50% health). If Roost is out of PP and Pelipper is under 50% health, you must use Ether on Roost (this is the only exception to the healing items rule). Solo an Elite Four member.
If _______ is Mienfoo, it must learn Jump Kick and High Jump Kick ASAP, and must lead every battle with either move. Solo Marlon's Jellicent to evolve.
EDIT:
CowboyJake take
Magic! the Gyarados.
You have no restrictions as a Magikarp, because that's not allowed. However, once you evolve your Magikarp (you MUST evolve at level 20, nice try), things start happening. Magical things.
Firstly, as a Magikarp its dream had always been to climb waterfalls, and it kept splashing persistently as it trained itself to do so. As a Gyarados, well, a 23-foot serpent can climb whatever waterfall it wants to, as long as it just keeps splashing higher and higher. Thus you must always keep Splash on your movepool. In addition, learn and keep Waterfall when you get it.
Now, you may be asking, this challenge is easy, right? After all, once you get to Couriway Town, you get the almighty Gyaradosite! However, with Mega Evolution comes a price, and the price of Gyarados is quite severe --- while you have Gyarados in your party no other Pokemon may hold a Mega Stone. It also demands being in your party for every gym battle, regardless of whether it intends to participate. In addition, Gyarados must always hold its Mega Stone, but may not Mega Evolve unless it survives an Electric-type attack from a Pokemon no less than ten levels under Gyarados.
Oh, and speaking of Electric-types, you must solo every Electric-type Pokemon you encounter with Gyarados (after evolution of course). And yes, this includes Clemont.
TL;DR:
-Evolve at level 20, no later.
-Keep Splash forever, also learn and keep Waterfall forever.
-While Gyarados is in your party, no other Pokemon may hold a Mega Stone.
-Must be in your party for every gym battle.
-Once you get its Mega Stone, that is the only item it may hold.
-May not Mega Evolve unless it survives an Electric-type attack from a Pokemon no less than ten levels lower than it.
-Solo every Electric-type Pokemon as Gyarados (NOTE: obviously this solo is void if Gyarados is boxed)
-Solo Clemont.