With Sekiro, although mastering the deflect system is the "intended" way to approach the fights (each enemy move can be countered in some way) there are shortcuts to go around it in case your skill or reaction time are not up to the task: the prosthetic arm upgrades. Some of these can be used and abused to trivialize some of the hardest bosses: the upgraded firecracker is usually devastating and can be used to cancel many enemy moves you're having trouble with, while there are other more "situational" tools such as the whistle that are very effective against specific enemies. I'd say this is the most "RPG-ish" aspect of what is basically an action game, while Souls relies much more on traditional RPG mechanics such as level-up or weapon upgrades (and maybe grinding).
Personally I finished the game and the optional boss-rush gauntlets using mostly deflect/mikiri because I liked the system and I can be quite persistent (even if my reflexes were never very good), but if you're willing to experiment with the prosthetic tool you will be rewarded with much easier boss battles.
As for the games I am determined to beat, I'd say the original X-Wing and Tie Fighter. I had both games when they were originally released, and was able to "beat" them by cheating my way through every single level... which was really unsatisfying, because I really liked the games. Now that I have mint copies of both (floopy versions, just like the ones I had) and era-correct hardware, I really want to take the time to redeem myself, but I'm kind of scared because they were quite difficult, especially due to the braindead CPU-controlled allies that you're supposed to protect but end up throwing themselves in front of a star destroyer.