Age of Empires II: Definitive Edition (PC) - Achievement - Inspiring Poet
Convert all Rajas before the Ghorids attack in Prithviraj #5 on Hard difficulty.
This achievement triggers once all Rajas are converted.
Although you are timed, this is not a difficult achievement, even on Hard difficulty.
I would recommend converting the North-most Raja first and make your way South, stopping to convert the remaining Rajas along the way. Once you convert a Raja, all troops in that respective city come under your control. You don't need to worry about resource management for the purposes of this achievement. Just keep moving your forces forward and try to use them as a shield to protect your Monks whilst they convert the Rajas. The Rajas aren't particularly deep inside their settlements as their settlements aren't huge. and the second and third especially, can be converted from outside the wall.