Never played this but that boss doesn't look fun lol
Can't really honestly point to 1 super hard boss personally that beats all bosses. I've beaten a lot of hard bosses.
I will say the most challenging boss I've faced recently was Rawr! in Fallout New Vegas.
I'm pretty beast on Fallout games so I'm usually on very hard difficulty.
Rawr! is the strongest Deathclaw in fallout and you're trapped in a quite small area near him. Basically 2 hits and you're dead. Thankfully after a few goes and with help with VATS I owned him.