I Agree, as I said, different people have different playstyle, once I know the game well enough playing through it normally just becomes too easy and mundane, it's more fun for me to try and complete different challenges that I set for myself, it wasn't so much for Demons Souls but for Dark Souls I did tons of challenge runs (my favourite being naked no shield big hammer & fang boar helm only run ). It's good that Dark Souls II actually take notice of challenges and added in alot of artificial difficulties like Champions convenent and bonfire ascetics, but for the most part damages are irrelevant since I usually got one shotted anyway during my challenges, I wish there is an enemy randomnise mode though.

I haven't had any trouble with some bosses, although there were a few especially ones that required magic to heavily damage (Around there i just summon some people), but the one who i had the most trouble was definitely the Smelter Demon i died around 7 times he was too fast for me. The easiest ones were the big ones i would just roll around attack twice roll again repeat. The one that i am expecting trouble with next is the King (Luckily i did not use any Primal Bonfire boss soul) so it should be easy with two other guys.

Maneaters were quite easy for me, especially with magic.

Ruin Sentinels were impossible for me without help.The range of their attacks were insane and took away shit ton of stamina with blocking. And even when i managed to kill the 1st one on the platform above, the 2 others were impossible they were always close to eachother and given how huge their attack range is, I couldnt even get close to them without being hit.

Rat Authority is something similar to Great Wolf Sif fight, except he is little bit bigger, more agressive and you cant go under him for easy hits and he hes a small army of rats who give you toxin when they bite you which drains away your HP exceptionally fast

well, EVERY boss is easy with magic (that's why I don't usually use magic builds), and if you know what you're doing and your character has a good damage output to kill the first one quickly then they're not so hard, but they are pretty hard for low level melee characters which is what I almost always go with.

as for sentinels you really shouldn't try to block them, try practice dodge and stride, it's quite easy for me to dodge or simply walk around their attacks after just a few tries, most of their attacks missed me completely when I simply shield hugged them, if you're blocking then you won't have enough stamina to attack after their swings, when they're close together, walk right towards them to bait their swings then roll back immediately, it's only safe to attack when both of them have finished their attacks as they need about 1.5 second to recover from attacking animation, the key is to be patient and attack only after they have missed their attacks then back off right away,  you only have a small attack window once in a while so expect a long battle.

Rat King does sound like a hard one though, maybe I am too quick to judge after all then.

Update: Smelting Demon killed my like 20 times because I tried to bum rush it to keep Lucatiel alive for her quest . The bad news is I wasted around 150K souls on weapon buffs and fixing life protection rings and still didn't get it (eventually gave up and killed the boss on my own), the good news is now I can literally dodge most of its attacks by sound alone +_+

Other than that stupid shenanigans, my boss kill count are close to 20 and still haven't seen anything hard, though the new Gargoyles would potentially be a tough one to beat on a soul level 1 run ~_n