Why is Fizz's damage uncounterable? 300 MR vs 400 AP

(Special note: Fizz did not have Void staff, his only magic pen came from runes. His only defensive items were Abyssal scepter and Zhonya's hourglass, which somehow made him tankier than me at twice his max HP.) Does he have some secret true damage formula? I just lost a game as a Darius with 3500 HP and 300 Magic resist against a Fizz that had between 400 and 500 AP - and he would literally 2-shot me. Why is Fizz's damage unique in this regard? Why is he completely uncounterable compared to, say, 600 Ap Syndra or 1100 AP Cassiopeia? Because against any other AP champion in the game, 180 MR is usually enough - they can barely scratch you, occasionally take down half your health if they unleash everything they have on you - but Fizz? Fizz takes you down from 100% to 0 in his WQ-E combo, banshee veil and all. Why? And am i forced to buy and learn how to play Galio in order to counter him when he doesnt get banned?
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