Getting stuck on a non-optional boss?
-Bloodborne: strategize with different weapons + grind to increase your stats + upgrade your weapon + change your outfit for better elemental/physical resistance + experiment with different runes and gems. Also, you have 20 doses of healing items versus the 10 of Sekiro.
-Sekiro: get good. Only way to increase your attack (at least until you reach Fountainhead palace and complete the mask) is by beating the very same boss you're stuck with. HP upgrades are finite and require you to beat mini-bosses that are sometimes harder than the main bosses. Consumables that grant extra resistance have lengthy animations that can get you killed mid-battle if you want to re-buff.
Bloodborne is way more forgiving than Sekiro and is a breeze once you find the right build for you. Sekiro, on the other hand, basically forces you into one specific playstyle with no choice in terms of builds.
Even if you grind non-stop to complete the entire skill tree, it won't help you THAT much to defeat the bosses (unlike Bloodborne in which you can grind to Level 100+ and breeze trough the main campaign with a 50 Bloodtinge build using Chikage, Bloodletter and Evelyn.)
*raises umbrella flame shield*.