Age of Empires II: Definitive Edition (PC) - Achievement - Malleus Scotorum
Kill William Wallace before Robert the Bruce betrays you in "Hammer of the Scots".
The level begins and you must defend your villagers from waves of light attacks. Once the initial timer expires, the attacks stop and you can research 'Warwolf' at the Castle your given control of. This shortly after produces a hero Trebuchet unit. The idea is that you destroy the gate and kill the enemy units guarding the Castle (the Castle and towers don't attack you FYI). Once you do this, the build phase of the level begins and so too does the countdown to Robert the Bruce betraying you as he is sent away from your camp - There is no visible timer for this.
There is a way to do this achievement without any time constraints however. You'll notice that before you take the Castle, you are trapped from escaping your starting camp by barricades which disappear once you take back the Castle. However, you CAN use Warwolf to attack the ground where the barricades are to destroy them, so that you can explore and kill William Wallace while Robert the Bruce is still at your camp (and not busy preparing to betray you).
Once you do this, you have all the time in the world to kill Williams Wallace. He is located South of your starting camp across a river and is guarded by a sizable force that includes Onagers and towers. You can actually defeat his guards with your starting army. All you need to do is snipe the towers with Warwolf and lure his troops towards you a few at a time, slowly killing his forces so that he has nothing left to protect him. Your Longbowmen excel at this, but watch out for the Onagers - Try to take these down quickly with your cavalry / hero. It's best to attack from the North-East. Once enough of William Wallaces forces are taken out, move in for the kill and trigger the achievement.
You can do this efficiently enough to still have enough of an army left to take the Castle near your stating camp and continue with the level. Just remember to use the Monks that you're given to keep your troops in good health.