My best accomplishment so far is still Platinum'd Bloodborne, Dark Souls 1 PTDE/2 SotFS/3, Sekiro, Elden Ring, and Demon's Souls (PS3). The most fun Platinum is Bloodborne (Chalice Dungeon is genuinely fun side content out of all Plat run here), and Demon's Souls is the most miserable.
Finishing Hard Mode in Armored Core: for Answer is also not an easy task, as the last Hard Mode mission is 1v5 scenario, you cannot let your AP be below 50% (or 30%), and all of them have extremely good coordination. "Occupation of Arteria Carpals" is the mission.
I never beat Armored Core 4 Hard Mode because FromSoftware forgot to balance the S-Rank requirement, and some of the easy mission in Hard Mode is straight up harder than the hardest mission. In Normal "Swarm of Red Eyes" you basically Tower Defense from drone with 0 risk getting damage, you just need to let no Red Eyes drone get past, and you get S-Rank. But in Hard Mode, enemies spawn and any single hit void your S-Rank immediately.
I've done Armored Core 6: Fires of Rubicon all S-Rank, but it's honestly the easiest out of all Armored Core when it comes to S-Rank requirement. Armored Core 2 and Last Raven have stricter requirement, which basically Time Attack (beat it fast).
Kneeling-Only Challenge in Armored Core (PS1), the least challenging Armored Core run. But in Project Phantasma this challenge is miserable because of Phantasma's HP and possibly DEF is too high.
Beating any ending in Armored Core: Last Raven Portable is achievement by itself, but S-Rank Zinaida ending is borderline insanity. It's gauntlet with single bar of AP, and then you pit against Infinite EN bullshit. The PS2 version is fine.