Definitely the didact and Tarturus. Various Mario enemies (besides bowser) such as Bouldergeist which is a ghost boss from Super Mario Galaxy.
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Sephiroth in kingdom hearts 2. So much rage and many attempts.
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These four gentlemen on my SL 10 playthrough. I don't think I'm in shit, [i]I KNOW I'M IN SHIT.[/i]
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The hydra from Spartan Total Warrior.
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Lucien from Fable 2. I still can't beat him.
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Aliens colonial Marines has two. All though that maybe because aliens gave me nightmares when I was little.
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Bloaters in TLOU on survivor. Especially if you do not have the flamethrower.
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I just got to Apprehension on Black Mesa. ICTHYOSAAAAUUUUR.
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The Uber-Necromorphs in Dead Space 1 and Dead Space 2
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Helicopters in Modern Warfare.
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Death claws