I've played a lot of games, and very few have I stopped playing based on difficulty, and this one isn't really "hard", but.. well I'll explain.
The entire objective (minus going through the bosses/levels) is souls. Each time you kill a person, you collect X amount of souls. If you die, your pile of souls stays on the ground right where you died at, and if you reach them, you get your souls back. If you die before you reach them, you lose all your souls. All the enemies/monsters re spawn when you die and you have to start all the way back.
There are checkpoints few (very few) and far between. I died right on a boss, this means I have to fight every single monster to get back to the boss, about a 15 minute treck back to the boss. The bosses are insanely difficult, so the game plays like this:
1. Reach checkpoint.
2. Die 10-15 minutes later and all your souls drop to the ground
3. fight your way back to your souls where you died (or die before you get there and lose all your souls)
4. Die again
5. Rinse and repeat until you finally beat that boss/area
6. die again in another section and repeat all the above again and again and again until you pass that section
The game isn't "hard" it's just extremely frustrating and not worth the trouble of actually playing it. You fight the same mosnters over and over and over again down the same path over and over and over again to 'maybe' reach your souls (if you don't die on the way back to your body) just to fight that same hard/difficult monster again and to die again, and have another nice 15 minute fight back to the boss if you can live through it.
The difficulty/challenge of the monsters is fun and enjoyable, but the game is designed to straight piss you off, and is not worth playing it in my opinion.