Put a siege machine in your army. You could train two stacks with each one catapult. One stack will later become the garrison of Paris and the other is the attacking force. Once the mission of the pope is over attack with both stacks simultaneously. Don't build siege equipment but use your catapults to blow a hole in the wall. The other army won't be much of a help so you need to have one strong attacking army, strong enough to do the job. Once Paris is occupied with your second army retrain your first army and train a second garrison. You have now probably some time because the pope has told you to back of for some time and after he is satisfied ..... well start over again with the next city.