This achievement was glitchy in the past, and I couldn't do it, but it was recently fixed.
I just unlocked it with a combination of the solutions posted above.
First I put all my villagers on stone mining, until I got the 650 stones. In the meantime I built a monastery, and created 2 monks, then I sent everyone to Tabasco where I dropped the castle, and built another monastery.
Go with the left route highlighted by @Mac10goesBRRRT, just in case.
https://imgur.com/a/EDjN5XGKeep building your economy and getting the upgrades to the castle and units.
Use your monks to convert units from blue, and when the galleons start appearing, send those units and let them play interference to draw the galleons fires, and then sneak your monks and convert the galleons.
Rinse and repeat until you have a strong economy and start rolling over red, and then blue. I kept villagers in Tabasco for when the castle needed repairs.
Here is the location of my castle, I hope this helps:
https://imgur.com/a/zKc71kQ