The game is designed perfectly. So far, I'd argue that the hardest part of the game is the very beginning of it for the following reasons.
1) You are new to the game, so you don't really understand the controls, nor are you proficient.
2) The base weapons, without upgrades are terrible for the mini-bosses. And because you are new to the game, you don't understand that a mini-boss is optional and you can go back to it. So you keep trying persisently, becase you are not proficient yet, you die, die, die some more and get discouraged.
3) Your HP and stamina is very low, same with all your defenses (phys,mag,light,fire)
The game is designed so well that as you upgrade your gear and get better, it happens at such a rate that you become better skilled at the game.
Black Knight in the undead burg, for instance, with the Morning Star unupgraded with a regular shield. Well, if you are not proficient, the Black Knight will do his three hit combo, deplete endurance and take half of your life away. One quick mistake and he swings again, you die. Because you are new this mistake happens.
Black Knight requires probably 6 back stabs with the non-upgraded morning star. Most people will make mistakes and will, again wipe.
Eventually your gear catches up and you can two shot them easily (sometimes one) and those are the even tougher ones. Part of you thinks it was all geared based, until you realize that you can now parry with a high success rate on a regular basis. Yes, your gear got better, but you got better.
So after paying NG+ and knowing how easy it is, you go back and start a new char. Your FLY through the game. That black knight? Yeah, your back stab or parry him 3-5 times and he is dead. You didn't get hit once. You kill Havel with absolutely no difficulty. What changed? You got better at the game. The gear and knowleged increased at a rate which allowed this to appear seamless to you.
I am thoroughly impressed with the game. By far the greatest game I have played in a really long time.
Now, are they still hard parts in the game? Yes - But the difficulty is a result of not planning your attack, or being careless (haste makes waste in some parts of this game) with it. I can't tell you how often I am switching spells/pyromancy/miracles/weapons/shield/rings in the game to assist with some unique situations. Game is awesome.