Alright this took me a while and afterwards I realized i could have done it a lot quicker.
There are a few steps you have to take
-NEVER convert villagers
-I also don't recommend upgrading the monks so that their conversion kills
0) Make sure to use the Save Game system
1) Convert crema
2) Scout as much as possible
3) Wait for Cremona to build a market (10-20 min)
4) Build a ton of trading carts
5) Get a decent army
6) Upgrade that decent army
7) Destroy the west gate of Milan
8) Have 20 priest beeline it to the Cathedral
1) Convert the entire town of  Crema. Kill all villagers and their town centre but convert all houses, markets, blacksmiths etc.
2) Get your Scout across the river and explore, you'll notice that the north and east gate of Milan are guarded by towers. West gate isn't
3) Keep watching Cremona so they build a market. Otherwise you cannot trade and have a gold income
4) Just keep trading most of your gold into wood. I recommend at least 30 trading carts. You'll be trading a lot of resources so the more gold income, the better.
5) I recommend 20-30 infantry. I couldn't convert a stable and wood went into my 6-8 Fireships to counter  River Guard warship navy.
6) Definitly upgrade through the barracks and the blacksmith
7) By now you'll have a decent army and you can see the west river branch right in front of the west gate. Make sure to start building your 20+ priests. The gate will take a while unless you're able to convert a trebuchet or cannon. The oganars will kill your priest so I recommend rushing them with infantry
8) Once through the gate, make sure to heal up and have a good army. Put your army in the middle of the town to draw 80% of Milan's army. Make sure your priests follow close but have them beeline it to the cathedral. It will only take about 7 seconds to convert the thing and you might lose half your priests but once it's converted you achieved victory AND this achievement :)
Extra's / optional
-Upgrade your transport ship
-To help with food, build a handfull of fishing ships
-If you need more houses, the south east will have a few next to a dock. You will get raided by cannons so make sure your priests are spread out and your army stays on the move
-It took me about three hours because I went for a lot of optional things like taking over River Guard northern defense. They do have a barracks there which makes it easy to produce extra army. If you do a more lean run, just get the numbers in
-Always have some priest with your army for healing