Steam achievements question (Total War: Shogun 2)

Hello! I apologize in advance if I'm asking an extremely stupid question, but I was poking around in Google, but for some reason there is no answer there :) So, tell me, achievements for historical battles in the Sunset of the Samurai such as "If you don't leave - destroy any retreating enemy unit" you can earn only on the maximum difficulty or the game can bug at such moments? The fact is that I completed such a task in the first battle and demolished one of the retreating detachments cleanly, but I played at an average difficulty level. Nothing counted. However, my friend was earning a different achievement in a different historical battle, but he definitely had the maximum difficulty. Thanks in advance :)