Final Fantasy VIII - Ultimecia was hell for me, but I pulled it off. I made the mistake of leveling my characters to 99, which scales up the difficulty and strength of all the enemy monsters (like wtf, the point to grinding is to avoid that problem?). Essentially I made the game as difficult as possible unintentionally.

Getting through Sans on a Undertale genocide run is no easy feat either. That took me several dozens of tries. GET DUNKED ON!

Lastly, the original Metal Gear Solid, I found, was not an easy game. Fun? Oh, definitely, but very hard. And that was pretty much the only MGS game I really played.