Back in MoP, defeating one of the legendary pet battle creatures used to give an exp reward equivalent to that of a level appropriate quest. If you had a solid team that exploited the weakness of one and a lot of bandages, you could grind really fast. I got one pet battle down to sub 60 seconds, which is great for a quest reward’s level of exp.
They eventually nerfed that though and I haven’t tested pet battle leveling since, but I know it was viable for a while.