You are thrown into an arena with nothing on you.
You have to fight each pokemon from weakest to strongest stat wise. (Wishiwashi->Zacian-Crowned)
Use https://play.pokemonshowdown.com/teambuilder, anything goes, add Pokemon, then click BST twice to get the list. You have access to this information inbetween rounds. Also after every fight, you get completly healed.

Here's the gimmick : every 5 rounds won, you can chose one of the Pokemon you've beaten as an ally. These Pokemons can stack (if you have 5 allies, you can chose to use all of them for the fight), however they can only be used once. You can chose to not use any Pokemon despite having many allies in your inventory.
If the pokemon has multiple forms, you can chose to use another form instead of the one you fought against (example : rotom, wishiwashi, megas, etc).
If the Pokemon is part of the family of one of your top 10 favorite Pokemon, you can use its one more stage evolved form (Bulbasaur->Ivysaur) or use it for 3 battles instead of 1 (can't do both).

You lose when you get killed.

What is your strategy?
Who are your allies?
What Pokemon ends your run?
How far do you make it?