under those rules pokemon win
“Regenerator restores ⅓ of the Pokémon’s maximum HP upon switching out.
This effect will occur if Regenerator is the Pokémon’s Ability in battle when it switches out and is not suppressed; a Pokémon that obtained the Ability with Trace will restore HP when it switches out, whereas a Pokémon which has Regenerator but has been affected by Gastro Acid will not. Heal Block doesn’t prevent Regenerator from restoring HP.”
simply put, you could switch between pokemon with this
switching out doesn’t cost any pp, meaning that one could switch between them infinitely given they can tank a turn of lion attacks for 1/3 of their hp
now given that we have this infinite loop, the next step is entry hazards
all you’d theoretically need to do is put down something like poison barbs/spikes/stealth rock/sharp steel then loop till all lions are vanquished
given that we have access to nearly all pokemon, the amount of setup you could pull off would be insane
side note; if the lions don’t have any “dark type moves” then you can also just flip between any ghost type and a chandelure with flame body