Age of Empires II: Definitive Edition (PC) - Achievement - He Won't Be Back
Assist King Bela and defeat the Austrian Duke Frederick in the fifth Kotyan mission "A New Home" before 45 minutes have elapsed.
The first think to note about this mission is that you get a choice to begin with. Travel back to Bulgaria or head south to help King Bela. For the purposes of the achievement, you must head south to help King Bela. You'll note that there is a secondary objective to find a destroyed Castle. As you exit King Bela's base to the South, take an immediate right and you'll come across it as well as a few villagers. Also note that you only need to defeat Austrian Duke Frederick before 45 minutes have elapsed - No one else.
Start building up your economy quickly - With your main aim to get a sizable cavalry force and Trebuchets. There is another objective to capture bases for King Bela. This can be done by damaging the tower in the middle of each base. Be careful, as he can also lose these bases if the tower gets too damaged again by the enemy. As of the date of this recording (27/09/2020), there is currently a bug in the game which is reflected in this video - Line of sight of the ally doesn't work correctly, although the Age of Empires team have confirmed they are working on a fix, hopefully in their next quarterly update. This makes things a little trickier at present as King Bela will attack the enemies and it's helpful if you go with him, but I was unable to see when / where he was attacking.
The more bases you capture for King Bela, the stronger he will be. He will begin to push Austrian Duke Frederick before the 45 minute mark, so it's important to try and keep an eye out for this and assist him to make sure you complete it in the time allowed.