I just finished Sekiro. The last boss took me around 4 hours across 3 days and... honestly, it's not really that difficult after all. It's just that it takes a bit to learn the game, and learn not to play it like a Souls game.
Anyway, it's satisfying as all heck and I immediately started new game+. Tenchu 1 was the first game I ever owned myself so playing Sekiro felt very nostalgic, especially with all the nods and homages to that series (some music tracks were even composed by the Tenchu guy). Hope there's a DLC coming.