If we're making the assumption that the lions have a human like intelligence and a near hive-mind amount of cooperation, then the we can also make the same assumption of the pokemon. A Victini, Xerneas, and Gensect working together could really wreck the shit out of 10 million lions.
If we're making the assumption that most of the billion lions are in a 98.5 million square miles (or about 255 million square kilometers) radius Yveltal going into a premature cocoon would wipe most of them, and IDK Rayquaza could just take pot-shots from space taking out the rest of the...