Some of these things are technically difficult while others just try your patients (or both). I will divide them up accordingly.
Completionist:
- Completed the entire Demon Compendium 100% in Shin Megami Tensei IV. (Here's a reference link [https://www.neoseeker.com/shin-megami-tensei-3ds/faqs/1588425-shin-megami-tensei-iv-fiend.html] I found. May not be obvious to those not familiar with the game how mind numbing this process was compared to a modern Persona game or even some other SMT games). Also 100%'d Persona 4 and 5's compendium, but they were infinitely easier.
- Collected all original 151 Pokemon before OG Gold and Silver came out (cheated a Mew though). My little brother deleted my save so my certificate is gone, but most of my old Gen 1 mons are on an old Pokemon Stadium cart and survived the save file purge.
- Completed every major questline (including all factions) and collected every Daedric artifact in every TES game between Morrowind, Oblivion, and Skyrim (except Bloodmoon main questline).
- I have a metric fuckload of rare mounts, transmogs, pets, etc. in WoW, but my collection is honestly nothing compared to some people (not currently subbed).
Technically Difficult:
- One-shot a small number of notably difficult Soulsborne bosses through a combination of sheer luck, well-suited build, or by quickly figuring out a strategy. Probably the most notable of these is Laurence from the Old Hunters DLC.
- Beat Demon of Hatred on my first try in NG+! Also, the final boss of Sekiro only took a handful of tries on NG+ (my first kill took about 2 hours).
- Survived Nightmare King Grimm in Hollow Knight.
- Beat MOST of the difficult SMT: Nocturne bosses on my first try, including most of the Fiends (including Daisoujou but not Matador), Beelzebub, Metatron, and Lucifer. The hardest bosses for me were Mot and Mitra in the Diet building IIRC. To be fair though, I did do some research on Lucifer beforehand and had a pretty good build going into the fight (also took two Dark Might demons since the Lucifer fight is always no Kagutsuchi).
- Beat a decent amount of super bosses/difficult end bosses across various JRPGs (most first try). Some super bosses are really easy to over-prepare for and steamroll, so I will just include the ones that are not easily subverted/cheesed (at least that I am aware of): every time-loop optional boss fight in Bravely Default, true final boss of Octopath Traveler, Margaret in Persona 4, the Twins in Persona 5, Yiazmat+Zodiark (but not Omega YET) in Final Fantasy XII, all the fiends in both SMT IV and SMT IV Apocalypse (Twisted Tokyo bosses), all of the DLC bosses in SMT IV, and maybe some others I can't recall atm.
- I have Cutting Edge achievements for Ghuun and Jaina in WoW.
- Have done some self-handicapping challenge runs of RPGs I love but none of them have been too ridiculous. One of the most fun ones was playing through all of Kotor with just me as a melee jedi and T3 as a skill monkey. Malak's apprentice ambushing me on Korriban was BY FAR the toughest part of the game pseudo-solo.
- Beat Diddy Kong Racing.