Barbarossa 3: Pope and Antipope
The easy way
Have your 4 monks immediately get on the transport ship and bring your horse knight with you. Then sail southeast down the river, research fervor, Redemption and Sanctity. Ignore the enemy galleys and quickly get to the end of the river and get 4 monks off the transport ship.
Now all you have to do is wait for the enemy villager to exit through the castle gate which will open, or create a distraction by having your knight attack the castle wall which will cause the swordsman to exit through the castle gate, quickly use this opportunity to sneak 4 monks inside the fort and quickly head over to the cathedral and quickly convert the cathedral with 4 monks before the catapult can attack you and you should win the campaign.
The challenging way
Start by scouting aggressively on land with all scouting assets. Monks are actually decent scouts, except that they are slow and vulnerable. Guard the monks with your ground troops. The ships are at present your weakest link, and they should hide.
Pick off Crema's villagers with your monks as you find them. Collect the sheep. If you act promptly, you should get 10 sheep. When you find the relic, leave it where it is until Crema is pacified. There will be a brief skirmish with Crema's home guard. Put your knights against their archers, and your pikemen against their pikemen. You can heal your damage. Crema is a piece of cake. You should be able to get about 10 villagers before Crema surrenders. Maybe you can get more in total if you take less up front.
Your immediate priority is Redemption, so you can take over Crema's buildings, but if the River Guard attacks, you will need Houses and ships.
Build up as quickly as possible. Focus on the economy. Limit your initial military expenses to five cavalry archers, two War Galleys, and a castle, or the equivalent. That's plenty to deal with any sea or land attacks that you will face, actually in this entire scenario, because your own offensive will develop so rapidly that you won't need to defend. Of course, you will probably build more defenses, because in some sense, while most of your economy is allocated to the main purpose, a significant fraction of your economy remains available for general purposes, unless you ruthlessly re-allocate villagers on an ongoing basis. Frankly, it's not worth the hassle, and you actually don't know a priori how successful your plan is going to be. Bodkin Arrow and Ballistics are important.
Go Imperial as soon as you can. Start building Cannon Galleons. (For some reason, you get Chemistry for free.) As soon as they are available, start nuking the River Guard. Five Cannon Galleons plus five Galleons is a massive-a fleet for purposes of this scenario. Wipe out enemy docks as a priority, but really, raze everything to the ground. Very soon, all enemy offensive potential will disappear. Your home areas will be absolutely safe for the duration of the scenario, because you control the seas absolutely.
Your ally Carcano does very little in this scenario. Accordingly, feel free to take almost all the stone on the eastern landmass; Carcano won't use it effectively, but you will. You can leave them some gold, because you won't need it all, and you can expect to get a lot more across the river.
The next phase of this scenario is the conquest of Milan. Nothing special is required. Raid an area. Wall off the pacified area. Repeat as necessary. That's literally all there is to it. Not. As opposed to your other two enemies, Milan puts up serious resistance. It will appear that, no matter how many of their units you kill, there will always be about 10 of their units trying to get you. You should see a few monks and medium artillery, and lots of heavy infantry, hand cannoneers, and cavalry scouts. Milan loves to run up and down the shore, and they get massacred by your light and heavy naval artillery. Eventually you need to push inland a bit, so you build a castle on the west bank of the river (behind a wall, of course), and then Milan's troops get massacred by your castle.
There are several special points that should be realized. First, Milan does not use much heavy artillery or any battering rams. Therefore a Wall Maze Complex is not required; a simple wall is sufficient, albeit necessary. Castles pwn anything Milan produces.
Having said that, you will need a maximum force, because Milan does have a powerful field army, and many of their units are actually good, instead of the usual pikeman/skirmisher/light cavalry junk that the AIs produce in extremis. Once you are at your population limit, start converting Milan's Elite Teutonic Knights and hand cannoneers, which, accordingly, you should not bother producing on your own.
You will not at first be able to raid Milan's villagers and cripple their economy, because, when your attack goes in, they will still be working inside Milan's wall. This will eventually change.
The easiest approach to Milan is from the north, because your ships can be used there. As you push forward, you will eventually be able to take out their north gate and replace it with your own gate. After that, you could continue to push into the city from the north, but it is more flexible to attack their other two gates one by one and seal them off as well. When Milan's three gates are sealed, your heavy artillery can roam the map with impunity. You will now be able to assess what is the easiest route into the city. Keep pushing forward, and pretty soon, all resistance will collapse.