Armored core 6 - Chapter 5 - Bring Down the Xylem - S Rank 

Difficulty definitely is a bit low side here for sure, maybe due to the fact that player went thru the most high amount of Bosses(Chatty stick, Carla, optional V.II, Smart Cleaner, Balteus) in finale among all 3 endings. And also the mission itself not being NG+ makes such difficulty scaling bit more plausible as loadouts are limited. Lore-wise though, I have a different (maybe overly dramatic) plot in mind! Walter was so shocked about 621's choice and couldn't think straight - become furious of the betrayal and death of his friend(s). During fight, he hears Ayre's voice and get even more furious, trying to shut the fire(Ayre) off for the last time. He therefore went full on to make 621 'pay the price' but since he's engulfed in anger and not sane enough, he made poor tactical decisions and moves - losing to 621. As he charges weapon in fury even after his AC destroyed - his head cooled, remembered the conversation with Carla: "I think 621 made a new acquaintance." ... "It is 621's choice to make." "Even if that choice contraries to our acquaintances' ?" After remembering that, he realizes that Ayre is 621's new acquaintance, and 621 made a choice as Walter himself said - a contrary one Carla asked as What-If scenario. Walter sees his choice to continue the will of his acquaintances is no different from 621's choice to go with Ayre. He accepts the 621's choice, finally answering what Carla asked before - finally setting 621 free from leash.