Ancient Dragon have a 1 OHK move doesn't make it 'difficult' it makes him terribly designed, much like the Bed of Chaos.

You and I come from either different gaming styles (Me from MMOs where if you stand in the wrong place, you die OHKO style) or different generations (I grew up playing Ghouls and Goblins where everything killed you instantly). Everything in almost every MMO raid is a one hit KO. Learn where to stand and it won't happen. Dark Souls is literally doing the same thing. They didn't invent the OHKO, it is a learning tool. Ok, I died... don't stand here. You can't expect not to die on a game that has the tag-line/motto: "YOU WILL DIE!" You call these fights poorly designed and yet, people are able to do them now, no cheese, no deaths(Verifiable on Youtubs). That is consistency and I am sorry to say that consistency is not "Poorly designed". Ancient Dragon/Bed of Chaos are easy when you learn the patterns.

Vendrick is also a complete joke of a boss tbh, I'm surprised you consider him difficult when all you had to do was hug his left leg and hit him.

Try fighting him on a blind playthrough first go. You know the kind? The one without a game guide or a wiki... so No one told you about the Giant Souls and you have 1 in your inventory... Good luck.

To me, it sounds like you want to claim that DS3 is the hardest of the series. It isn't - Most people(reddit, reviewers, etc) have already said so... You are a lone holdout, my friend. Maybe the DLC will change that and I am praying that it does. DS3 is definitely the best send-off I could hope for and I love the game. It is the best game of the series and I am going to treasure it for a long time... but I felt that the bosses were weak compared to the previous games.