Let the Pope have rome. Just take the rest of his provences. When he is pinned up in rome, attack. Kill as many men as you can, but let at least a few make it to the castle. Now you can just leave and let the few survivers have Rome. There force will be so small they will not attack for many, many years. It works even better if you can capture the Pope and drain some of there money.

Think of the Pope as a pet