Age of Empires II Definitive Edition Age of Empires II: Definitive Edition - The Missionary Achievement Guide (Useful Tips)

This guide complains how to get it in first mission from the El Cid Campaign.

The Missionary Achievement Guide

Select the El Cid Campaign.
Select the first mission: Brother against Brother.
Fight as the kings commands.
Gather your army and follow the road towards the middle of the camp.
Quickly set up your base near the walls of the enemy team.
Send some scouts to the left corner of the map to discover 4 Spanish monks along with 4 relics.
Get the relics inside the Monastery to farm gold.
Spam villagers and farm resources.
Attack the enemy and be prepared to defend yourself.
After blue's army is dead leave the Conquistadors to the gates (they can defend everything that is coming to attack) until you gather resources; King Sancho will eventually reveal the whole map by researching Cartography in this period of time
Advance to Castle Age.
Build a Church and a Siege workshop.
Send your rams to attack the castle.
Send all your other soldiers to attack the monastery to the right of Alfonso's Castle.
Make sure the church gets destroyed BEFORE the castle to get the achievement as King Alfonso will change the stance to Ally and you will not be able to steal his relic
Garrison the last Relic.
Deliver King Alfonso and El Cid to the fighting grounds.
The achievement will pop-out after the mission is completed.

Source: https://gameplay.tips/guides/5904-age-of-empires-ii-definitive-edition.html