Age of Empires II: Definitive Edition (PC) - Achievement - Offense Is the Best Defense
Defeat all opponents before constructing a Wonder in the sixth Saladin mission "The Lion and the Demon".
As of this video being uploaded (22nd November 2019, game version: 101.101.33059.0 4403463), there is what I believe to be a bug with this achievement. Many people, myself included, realised that the achievement is not as simple as it initially seems. Your goal is to defeat your enemies before building a wonder, however, once you defeat your enemies, the game ends and your are unable to continue building. While it doesn't seem the way this achievement was intended to be obtained, you can work around it by changing your diplomatic stance with Persian Outpost (purple) to enemy so that the game doesn't end after defeating your final 'actual enemy', but this is hopefully something the Age of Empires team will look in to / clarify in the near future.
UPDATE: The December 17th 2019 update has fixed many issues relating to achievements. This should no longer be bugged.