Age of Empires II: Definitive Edition

The path in this mission is very straightforward and the achievement is easy.
Blue and Yellow don't have monks so as soon as you've destroyed Orange you're fine.
I only produced cavalry so I could quickly get to the monks before they have time to convert my unit.
Save before each Orange settlement and keep track of how many units you lost to conversion.
Don't forget to keep producing cavalry when your ally gives you ressources !

Based on Standard difficulty:

Don't forget to keep producing cavalry when your ally gives you ressources !

1st settlement: 2 monks
2nd settlement: no monk
3rd settlement: no monk
4th settlement: two monks + a temple, so they can spawn more.
Bait their army by attacking the walls, most of them will come out and one of the monks will be with them. The other one is waiting deeper in their city. Wait until one of their soldier opens their gate and go through with your army and destroy everyone. I'd suggest to focus the temple quickly so that they don't create more monks.
With loads of calavry it was easy, they can't match you.

Now that Orange is no more you can just end the mission normaly.