Things to make the Hardest boss fight...

[Slash Dementia]

Multi-bosses are pretty hard, especially in games like Dark Souls where being surrounded basically means death.

The Bell Gargoyles kicked my ass endlessly when I first played the game, since I was still unfamiliar with the mechanics. Then I summoned Solaire and killed them in short order.

I still haven't summoned anything--I just don't know how. But god, that fight was brutal. Other than that, you still have to stay away from the edges. It took me many, many times to get through.

I think that a lot of times, the camera angle makes things harder. Some games, when you have a giant boss, the screen becomes focused on it, leaving you as a fraction of yourself on the screen. It makes you have to recalculate your attacks... That and quick time events. I hate when they're so sudden and force you into pressing a certain button, even if you don't really use that button throughout the game in battle.

 

[Rumpsteak]

Attrition - Simplified it is having the boss do more damage or lowering the players ability to regain health. This is the least fun way to do it but the easiest to explain.

Options - This is a variation on multiple phases. In essence it means doing different things either the boss or the environment will cause the boss to gain different abilities. You could do X action that causes boss phase A to trigger or you could to Y action and trigger phase B. The amount of design makes this hard to design as ideally the player would not see every possible phase during the boss fight.

Spawns that interact with the boss - One way to do this involves linking the spawns with the boss for somewhere in the region of 75% of the time they are alive. For each spawn that dies during this time the boss regenerates X% of health. This leaves the player with the choice of going for the faster boss kill and avoiding the spawns until the link drops or killing some of the adds to make the fight easier but take longer. This works well in conjunction with attrition mentioned earlier.