Mr. Freeze from Batman: Arkham City. He is an absolute pain to beat on New Game Plus mode. Unlike the standard difficulties, i was low on health and needed to do at least 10 offensive maneuvers to finally topple him.
The easy takedowns such as from behind, using the floor grates, glide kick, hanging from the ledge, disrupting his suit's mobility and the electromagnetic generators was easy enough, but the other four were even harder. Definitely the highlight of Arkham City for me.
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"I've seen things you people wouldn't believe: Attack ships on fire off the shoulder of Orion; I've watched c-beams glitter in the dark near the Tannhauser Gate. All those moments will be lost in time; like tears in rain. Time to die." Roy Batty