Age of Empires IV: Anniversary Edition

This is easier done with a boosting partner.

However if you wish to try to get this on your own (as I did). Try the following:
-Skirmish vs AI
-AI difficulty: Hardest (Hard may work as well)
-Map: Warring Islands
-Recourses: Very high or Maximum
-Starting age: IV
-Map State: Revealed
-Win conditions: Turn off Sacred
-Your Civ: Any (I went Delhi Sultanate)
-There Civ: Any (They were English for me)

When the game begins build some houses, train some villagers, build a few military buildings and pump out a bunch of military units. You don't need much of a military just enough to hold off <25 enemy units (I had the elephants which worked well).
Then build a Mosque (or your civ's equivalent), train a bunch of holy guys (Scholars or whatever) and get a relic. You'll have a relic on your island and there will be 2 more in the middle islands, you can do this with just 1 relic but feel free to build a dock and get the other 2 if you wish.
Also start building a wonder whenever you can. You'll ideally want this up before the enemy's.
Sooner or later the AI will start sending transport ships filled with military units. If you look at the enemy's transport ship you can see how many units are in them (max 16/ship). If <25 use your military to kill them all when they land.
When you can see 2 or 3 transport ships coming your way with at least 25 units in, get your relic-carrying holy guy to where the transport ships look like they're going to land and start pre-casting when the transports are near (i.e. click conversion before the ships actually unloads the units). I had something like 25 holy guys there just in case i mis-timed and they start attacking my Scholars, at least they'd kill the non-relic carriers first giving me a bit more time.
It may take a few tries and a bit of messing about to get the timings right, but when done correctly the transport ships will unload the units and they will be near enough immediately converted (I managed to convert 46 units in one go).

Like most AOE4 achievements, it may take a few mins to pop.

Note: It seems saving and reloading voids achievements, so this needs to be done in one go. However the AI will send plenty of transport ships so you'll have many attempts in the one game.

Edit: If you're going to downvote at least explain why

This one may take a few tries, but it is NOT that hard.

Set up a Skirmish:
Map: Dry Arabia, Micro 2 Player
Win Conditions: Wonder Only. Turn off Sacred Sites.
Civilization: Any. Russia might be easier for Warrior Monks.
Opponent: One AI Mongol enemy. Hardest if you can handle it. Easy will work.
Match Options: Starting Resources: Maximum, Starting Age: Age IV, Map State: Revealed

1. Queue up some Villagers and keep producing per usual. Though you could probably get by without much.
2. Quickly start these buildings:
A. Task a Villager to build houses until pop cap is reached.
B. Build 12 or more military buildings to pump out an army of melee unites. I used Camel Riders as Abbasid Dynasty.
C. Build two Monasteries to pump out Monks.
3. If playing Hardest, the AI will start raiding right away. As long as you are pumping out units ASAP you will soon overpower the raids.
4. Send Monks to gather Relics as soon as your monks leave the Monastery. Bring them to your base until you have them all.
5. The Hardest AI will build a Wonder very quickly and clump a large army around it. There will be no walls and minimal defenses because they are Mongol. Easy AI will take much longer to build their Wonder. Maybe 15 minutes. You can wait, or attack their unit mass in their base, or provoke them into the center of the map.
6. Approach the enemy with a couple of Monks carrying relics, about 4 Monks to heal them, and a small line of 6-12 melee units to soak up damage and take out the cannons before they wreck your Monks. Activate your Convert button on your Monks carrying relics while using your military to drag enemies close to the Monk. Convert a bunch at once to win!
The timing is tricky. Take out the cannons before they get in Monk range. You may not get this every time, but you will get it after a few tries. Good luck!

The easiest method to gain this is to boost with a friend or use a second account on a laptop/computer as I did!

Setup a custom (private) multiplayer game with your 'friend' with all the positive alterations; Max resources, revealed map, and so on. Team-wise, I personally had Abbasid VS English but it shouldn't matter too much.

To set this up:


YOU:
Build a mosque and then create around 10-15 monks, more is better. Move these to the center of the map to meet the enemy, have one or two of your monks go and collect an artifact. Place these bearers at the back of the group.

OPPONENT:
Simply create a mass of villagers and move them to the center of the map (not too close to the enemies monks though! I used around 35 to make sure.


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Now march your monks directly up to the peasant army (they're still potentially lethal and will kill your monks fairly quickly. Once your artifact monk is within range start the conversion and hopefully change them all at once! Achievement will pop shortly afterward if done correctly.


Tips:

- Have your opponent mash the STOP command on their villagers to slow down their villagers' attacks (they'll auto retaliate within range so you can't stop them, just slow them down!)
- Using mass monks stops your army from attacking their villagers and risk a lower conversion number.
- You cannot convert enemy monks... I tried!