Age of Empires II: Definitive Edition (PC) - Achievement - Friends, Countrymen, Lend Me Your Ears
Convert Afonso de Albuquerque without killing a single Portuguese unit in the fifth Francisco de Almeida mission "A Son's Blood".
You start without a Town Centre on this mission, so you can't build up your economy early on. You also start with two Monks and a Missionary. Get some of your Villagers mining gold and build a few monks to assist the ones you already have.
Once you have converted 10 of Afonso's Armada (Red) men, you will be able to convert Afonso de Albuquerque which is basically the end point of the achievement. However, what you are not able to do until then is kill ANY of the Red army. You can take casualties yourself - that's fine. Just do not kill any of his men. You'll need to micro your Monks as best you can and bear in mind that as you don't have a Town Centre, you can't advance to the next age and research Theocracy, meaning that if you convert a unit with a group of Monks, they will all have to regenerate their faith.
Once you have managed to convert 10 men, you should be able to lure Afonso de Albuquerque out of his camp by approaching from the North-West side, by the coast and attempting to convert him from outside the wall. You will take fire from a tower, but as long as you have enough Monks, you will be fine.