If it's a superboss, it should be 100% optional. I do agree that it should be very hard, maybe even borderline unfair. I would say something around that of Tartaros and ??? from IoR could be fair, maybe a bit harder. My opinion is that a superboss should be aimed at those hardcore players that want a real challenge and the fight of their life. It's optional for a reason. I can understand it being a barrier to softcore or just in general more casual players, though. As for difficulty based behavior? Sure, as long as it means something like:
Easy Mode = Boss is hard as hell and will definitely cause some rage and make the player want to book a flight to visit you
Normal Mode = Boss most likely will cause broken controllers/keyboards and make the toughest players cry
Hard/Hero Mode = Boss will make you either turn to drinking or put you in an asylum from all of the sanity you lost attempting it
I haven't really played too many games with superbosses in them, but using Kingdom Hearts 1 as an example, my first time playing on Normal, both Kurt Ziza and Phantom wrecked me. I had to get to level 70+ and try multiple times before I was able to win. Ice Titan wasn't really as bad, just annoying as hell. But Sephiroth...damn. Only being level 80+, grinding out for the Ultima Weapon and 2-3 hours of attempts (resetting every time I failed so I didn't waste items) allowed me to see the One-Winged Angel's demise. At first it was me failing against the Heartless Angel move. Later, I was getting wasted by his Omnislash. Then, just when I thought I had him beat, his ultimate attack, his projectile spam, his sudden Heartless Angels and constantly shouting "Dodge this!" put me back in my place. Mind you I was only 13 at the time and now I have much better reflexes and can read patterns much more easily, not to mention I know a lot more about the game than I did as a 13 year old. Honestly, that was a time beating Sephiroth at all made you the man, not just severely underleveled, with Kingdom Key and without getting damaged.