The trope you are looking for is That One Boss. [http://tvtropes.org/pmwiki/pmwiki.php/Main/ThatOneBoss]
My personal That One Boss was Bloody Mary from
Terranigma. I'm pretty sure this was due to a programming error that made her stats unreasonably higher than what they should have been for a boss of her level. As a result, if you were only at the kind of level you would reasonably be expected to be at by the time you got up to her, you'd find yourself facing a near-impossible battle where your attacks only do 1 or 2 points of damage to her- and she has HUNDREDS of hit points. Not to mention that she's hard to hit without taking damage, has attacks that are hard to dodge, and does a lot of damage. Sure, she was one of the few bosses you were allowed to use magic on (she had a weakness to Elec Rings) but your supply of those was limited and again, if you were only normally levelled it wouldn't be enough. Unless you were extremely skilled (and patient; doing it the hard way could take AGES of attacking, dodging and running) the only option you had was to level-grind your ass off.
The guy in this video is somewhat over-levelled (level 23), but almost all the damage he does comes from his rings. Mind you, he's also not fantastically skilled. But either way, Bloody Mary was
by orders of magnitude the toughest boss in the game. None of the others even came
close and some of them were actually pretty tough (the Security Robot in Beruga's Lab was a standout, but even that couldn't so much as touch Mary).
Julianking93 said:
I always had the hardest time on that first Dark World boss in A Link to the Past.
You know, that weird ass turtle thing with the big tail...
That thing.
May I ask why the Helmasaur King caused you so much trouble? OK, maybe it can be a bit obtuse if you don't know what to do, but how hard can it be to work out that you hit its mask with the hammer until it breaks then hit the jewel on its head until it dies? And once you've worked that out it's not like it's very hard to do.
Now
Moldorm on the other hand...