We've fought Old Gods before, and Garrosh is not even on that level. Plus Varian has a pretty solid record of kicking Garrosh's ass all over the place when they 1v1, so there's that. I'm pretty sure that most of the other powerful lore characters who could take him will be involved too. Credit for the kill is going to be something like "Varian, Vol'jin, Baine, Jaina, [insert rest of the list of NPCs here] , and a group of adventurers who assisted them."
Also, remember that Garrosh is only empowered by an Old God. It's not like we are fighting Y'Shaarj himself. Or, at the very least, we're not fighting Y'Shaarj at his full strength.