Going off the game
Okay so if we were going to use in-game logic, like if Lions were pokemon in the games, we could easily win this. 1 billion lions seems like a lot, because it is, but there are pokemon moves that can hit all opposing pokemon at the same time. Kyogre could use Water Spout, a 150 base power move from a pokemon with a special attack stat of 150 (for reference God’s is 120) plus it has STAB. Of course this is one pokemon though but many Pokemon could use moves that aid Kyogre’s attack, like using a pokemon with Helping Hand to boost Kyogre’s attack by an additional 50%
The lions likely couldn’t outspeed Kyogre as well. Sure we could give these lions items, perhaps a choice scarf to make them 50% faster but only able to use one move, but that’ll all be for nothing since we can also give Kyogre a choice scarf and apply Tailwind to it via Tornadus; who is garenteed to be faster thanks to his Prankster ability which gives priority to status moves.
Game wise the lions do not stand a chance, unless they have some busted stats and a brain behind it all to make it functional I do not see a scenario in which they win. This is only with a few pokemon too. Kyogre, Whimsicott, and Tornadus were the only ones needed so far for that scenario and that’s a near definite win for Pokemon. Add in a couple hundred more, with a good portion even knowing Helping Hand, and it becomes an absolute slaughter.
Going off “lore”
Pokemon have a far less likely chance of winning here without Kyogre’s “kill everything” button. If we were to go off of pokedex entries then it’s just insane. Pokedex entries are notorious for being crazy. Regigigas alone can move entire continents. Continents. He might not equate well into defense but if it comes raw power Regigigas alone can surpass them.
A billion lions sure is a lot but there are just some wacky pokemon. They couldn’t do jack shit to hurt Metagross for instance. It’s previous evolution, Metang, can apparently crash into a “jet plane” and get off without a scratch to it.
There are pokemon who beat all 1 billion lions together in brute strength, there are pokemon the lions can’t even damage, and there are also pokemon that the lions cannot even get close to (Magcargo). Infamously insanely hot, comparable to the surface of the sun. I’m sure the lions could deal with it but not after taking some serious losses and either before or after dealing with the legendaries, who can spam attacks out of the lions range practically indefinitely.
In conclusion
The lions, while having a huge numbers advantage, just don’t compare to Pokemon in game or using their dex entries/lore. I’m sure if you used the anime versions you’d be able to make some argument about it but the anime gets shit wrong about pokemon all the time so doubt that’d mean anything to anyone. Ash’s pikachu knocked out a Latios and like 20 episodes later lost to a level 5 Snivy, big whoop.
)