I haven't played Dark Souls, so i don't know how this stamina bar feels to play, but i would say it depends on how your combat system is designed.
In my opinion using the same resource for everything does not sound like a good idea. The Problem with this is, that you can't do anything, when the resource is gone.
My idea to this Problem would be make at least two different resources.
One for blocking, sprinting, rolling and other movement.
The other one for attacking (like Mana in many games) and an attack skill you can always use. So it doesn't matter what happens, you can always walk and attack with a weak move. But you have to watch the resources for better movement and attacking.
Ok, maybe one resource for both would work, but you shouldn't take away every possible action from the player. Thats why i would let him always a weak attack.
One or two bars shouldn't be a problem for casuals, if you explain what they do. But then the balancing gets very important. If the bars are too small it will be frustrating, if they are too bit, they won't matter.
If your only action is hit the Enemy with the sword maybe you should work at this before making a stamina bar. Running away from an enemy with 10% life because you can't attack him and you have to wait for stamina doesn't sound like fun.
For the Bosses. I always liked interesting mechanics more than resources.
If they are too easy, give them a few devasting attacks with a big indicator, so it's possible to dodge them. Maybe make them only vulnerable after such an attack because if they fail, they stun themselves or something like this.
Easy games can also be fun. And remember: you designed the fighting system. Maybe attacking and blocking isn't that easy for other players. Let a few friends try your battle system and ask them what they think about it. Probably they can give you better feedback than everyone here who is just reading your question.