there a lot of questions that needs to be asked before we can even begin to answer the 1 billion Lions Vs 1 of every Pokémon. for starter when are we asking the question, 1 billion lions would destroy 1 of every gen 1 Pokémon, assuming all the lion's attacks does neutral damage, along those lines the typing matters, as I said they could beat every gen 1 Pokémon assuming their attacks has always does neutral damage, if all their attacks count as normal type a single ghost type can solo 1 billion lions. also assuming lions does neutral damage then a single shedinja would defeat all 1 billion lions because only super effective attacks can damage a shedinja, however if a lion's bite counts as a dark type move than they could defeat shedinja's wondergaurd. also what counts as 1 of every Pokémon? do regional forms count as different Pokémon or the same Pokémon, and if they counted as the same Pokémon what form will be in the battle, as galarian mewoth has the edge over National Mewoth and Alonan Meowth as their metal body would be harder for a lion to get though. also how inclined are you to to believe pokédex entries, because magcargo would instantly beat all the lions just by simply BEING HOTTER THAN THE FREAKING SUN! also is PP real or is it just a game mechanic? because even with the almost 900 Pokémon as of gen 8 they just don't have enough PP to defeat 1 billion lions.
TL;DR: who would win 1 billion lions or one of every Pokémon? well that depends.
TL;DR: who would win 1 billion lions or one of every Pokémon? well that depends.