idk man, a billion is a pretty big number
Half-lives make it a lot less big. Also, apocalyptic powers tend not to care about large numbers, except in the abstract "that's how much I destroyed" way.
So, for people arguing the pokemon will be fatigued to death: the lions will fall asleep from raw fatigue, and then since that's a natural sleep, not inflicted, Darkrai just floats around and they're all dead from Bad Dreams.
Again, ALL pokemon. Many legends are walking/flying/swimming APOCALYPSES. Groudon/Kyogre, Reshiram/Zeckrom, and Dialga/Palkia battles threatened creation or parts of it just as COLLATERAL. Then there's the IMMORTAL Xerneas and the infinitely-reborn Yveltal. Lions die just from being in proximity to the pokemon for long. And given how tanky some mons are, it will be a while.
Mons that solo the lions:
Dialga (time dimension, jump to after they all starve)
Palkia (space dimension, just lock them in a pocket reality and let them starve)
Giratina (distortion world, just smash some stuff and create a calamity in the center of the lions)
Arceus (own dimension)
Rayquaza (it can retreat to where the lions will suffocate even if they could ladder that high, which would just plain crush the bottom layers LONG before then)
Primal Groudon (plateaus itself to safety, then just destroys them by manipulating the crust / tectonics)
Most water types + Dragonite (retreat to deep waters and rest, where the lions will drown if they try to even get that far)
Mewtwo (remember that world-threatening storm from the first movie? Also he has plenty of retreat options)
The Cosmog final evolutions (Ultra Wormhole to safety)
Yes, most of them are legendary, but remember that's allowed. EDIT: except for most of the water types
EDIT: And that's just off the top of my head.
Others of note:
Ho-oh (It's a phoenix: infinite lives and can rez others)
Regigigas (moves CONTINENTS [EDIT 3: so it can just move the field around into something lethal to the lions])
Flying Types with ranged attack (attack until nearly tired, retreat to a nest out of lions' reach)
Yeah, a lot of pokemon will be eaten, sure. But some of them are so powerful, it takes other pokemon to subdue them. And with Ultra Wormholes, the weaker mons can mostly escape while the walking apocalpyses just destroy creation.
EDIT 2: since all media is in consideration, the anime showed quite a few things that just plain wins. 1 lion, 10, a thousand, a million, a billion, a trillion, a quadrillion... it just doesn't matter. It's hard for a mere part of the world to end a world-buster. And in the games alone: A single member of the Creation Trio was enough to remake the entire world, only stopped by either the Lake Trio or by another of the Creation Trio (Giratina in Platinum). The lions have no answer to that, and even if you assume the lions beat EVERYTHING else, there's a few hundred bodies in the way, given Dialga or Palkia plenty of time to remake it absent the lions.
EDIT 2: sure, plenty of Pokemon would lose if not for the greatest of them, but... ultimately, the divinities > any amount of lions, no matter how large. And quite a few of my math classes discussed various forms of infinity, so... yeah.