As a final mission this is not a particularly hard mission. The only thing that might be hard about it is the bonus secondary objective which involves moving up in Ages very quickly.
In this mission you will have Amelia Black and Kanyenke at your disposal. You will last and move north where a Spanish town which hates the Circle of Ossus agree to support you. They state while you macro up they will defend you the best they can.
To support this I’d suggest building a fortress almost immediately. The fortress will take most of the hits and by the time it is destroyed you will have an army.
It should also be noted there is a survival aspect to this mission. If you survive long enough ‘America’ will have sent war ships to help you. The war ships will take out the fixed cannons located all along The Ossuary. On top of this if you lose ships you will just get newer better ones automatically.
The main objective in this mission will be to get high amounts of production and attack. The more anti-infantry cannons you have the better you will do in this mission.
There is not a lot of land in this mission meaning that there is really not a lot of gold, not a lot of wood and not a lot of hope. This means this is not a mission you can just wittle the computer down with. You will need a proper balance of musketeers and artillery to bust down his walls.