Age of Empires II: Definitive Edition

This sounds hard, but it's actually very easy!
The achievement pops as soon as you complete the objective, so it is also very fast, less than 10 minutes.
There are 4 raja's to convert, and my recommendation is to convert the one at the top first.
Convert him, and shortly after a 10 minute timer will start for the Ghorid's attack.
It shouldn't be a problem, first time through I had exactly 5 minutes left on the timer when I finished.

Immediately have your villagers go to work with some mining gold(train some more), and also build two Monastery's right away.
Train some more monks, and research their speed and HP upgrades, although not really necessary...
Take all your troops, set their attack stance to Stand Ground or No Attack Stance, and as soon as your Poet monk is recharged, bring the troops up to the 2nd little base to take the attacks while your Poet converts the next Raja. As soon as he is converted, all the troops become yours.
Set the villagers to work if you want, grab all your troops and your new monks, and head straight south to the third Raja.
He's easy to convert over the wall, and rinse and repeat heading to the 4th Raja in the east.
Just send all your troops in with No Attack Stance, with your monk party behind them and convert him.

Pretty straightforward, just go top, top middle, bottom, then right.