The fight takes place on a flat and open field
There is no in-fighting between lions or raid bosses
Following WoW rules, meaning the lions can hit things like Ragnaros, I mean, if a hunters lion pet can hit Rag, so can a wild lion.
There is no running away or hiding, everyone is out for blood. Spells like fear and such still work though.
Lions are max level
Who wins?
Raid bosses win. Overlapping AOE abilities
Raid bosses, Argus obliterates the universe if he gets a cast off.
Argus unmakes the battle field one billion lions and all… not to mention LK Arthas who is pretty much his horse (invincible)… Not even taking into account that for bosses like Y’ogg, LK, Gorak Tul etc every dead lion is essentially a puppet soldier of undeath.
Mighty mouse
I mean couldn’t Arthas solo all of them? One “defile” and they’d all be wiped out since they aren’t smart enough to run away.
Against Agrramar , I still hear the echoes of “Unworthy” wiping through our lfr group . 
Barring Argus’s “unmake the universe” crap, it’s one billion lions.
That’s a big number. I’d put it on the lions just for numbers alone. Arthas’s defile? Arthas ends up suffocating under a pile of millions of lion corpses . . .
Who wins?
That’s easy.
The Old God BiggaBaggaLagga casts mass disconnect.
/win
Kazzak one shots the Lion Raid…
KAZZAK IS SUPREME!
You forgot Lord Kazzak’s massive AoE wipe ability when more than 40 targets are in it’s range. There was no cap on targets. That ability alone will limit the number of lions that can engage the raid of bosses to only 40 at a time. Also… lions will wipe on 1 boss because they can’t defeat it… no frost damage. Also can lions figure out how to Safety Dance? What about Spine?
Do the lions have an interrupt?
Can we substitute max level lions for feral druids? Which for all intents and purposes are skilled competent lions.