There's not really that many frustrating bosses in DS1. Yes, there are bosses which take multiple attempts to learn, but the challenge is usually fair and not overwhelming or anything like that. There's a few bosses that I even dealt with in one shot without dying like Gwynn, Manus, or that Nico whatever his name was skeleton boss from the Tomb of Giants. The only two bosses that frustrated me beyond my normal tolerance was Bed of Souls because of janky physics getting the last jump right when you're about to attack the heart and Ornstein/Smough because they were genuinely tough. That's not to say that like...Seethe, Four Kings, and other bosses didn't give me some trouble, it's just that they weren't frustrating enough to get me angry like those two in particular were.
Well, that's the thing. When you say "multiple attempts to learn", that to me is frustrating. Meaning, I don't necessarily find having to die a bunch of times to a boss to learn his every move a great combat design approach, especially when you talk about a boss like Manus, who has so many different moves and attacks.
My ideal combat system would revolve more around a "system", kinda like real life fencing, rather than unique enemies that you have to learn. The system would have basic principles, if this happens, do this, if that, that instead, and so on. There is some of that in DS, but each new enemy introduces enough wrinkles that you can't just face them without dying and learning.
Also, for a dexterity build, there is too much emphasis on reflexes, as some enemies attack ridiculously fast (e.g. Skeletons, Hollow Thieves, Gwyn). I would prefer more emphasis on technique, which anyone can learn and improve, rather than reflexes, which are fixed.
