You can't do the boss again after you've beaten it. Why would you even lie about that?
... I dislike being called a liar, but I see others have already jumped all over you for not knowing what you're talking about.
So this game is not for the faint of heart, is it? Is it like Ninja Gaiden hard or just grind-the-doodoo-out-of-it hard?
It's... neither. It's definitely not a grinder. And while combat is complex, it's not viciously complex like Ninja Gaiden.
Rather, the difficultly comes from punishing you for your own mistakes -- but in a way that it always feels like you deserve it. It never feels cheap, in my opinion.
For example -- you get really used to that giant sword, and bring it with you into a narrow tunnel. Good luck swinging it when that skelly is around the corner. Or, you get over confident and chase that weak monster when it runs and get ambushed. Or you took one too many swings, not leaving yourself enough endurance to block. Or you decide you think you can make that long trek across a poison bog without going back for supplies. Bosses are elaborate, giant, and often very difficult the first time you meet them.
But in general, death is always your fault. You took too big a risk, or didn't have your shield up as you rounded a corner, or freaked out and rolled away from a baddie when you had your back against a cliff. And deep down you'll know, had you, say, given more thought to the blood stains on the floors/walls, you'd perhaps have realized danger was close.
Also, death is punished - you leave a bloodstain with all the 'souls' (read: gold/experience) you've acquired and not spent, and have 1 life to get back to that bloodstain or else it's gone. (This can seem traumatic at first, but really, even losing a ton isn't truly a big deal). However, some of those runs to get to a faraway, but important, bloodstain are the most thrilling/invigorating.
RE: invaders -- generally, I love this mechanic. There are some griefers out there that are over-equipped, but honestly, it's not common, and even then there are ways around that (if you can get invaded, you can also summon, and there's nothing like having 2 pros at your side when you get invaded by one griefer). The online mechanic is the most innovative approach to online coop/pvp i think i've ever seen. Also, once you defeat a boss you won't get invaded in that area anymore. Some invaders will kill you. Just try again.
The sense of dread you'll feel, however, when you're knee deep in a bog and you're quite lost, and get invaded, however, is... pure gold. Terrible, terrible, fear and despair gold. I wouldn't change it for the world.