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Seymour in FFX. Never finished the game because of the absolute massive difficulty spike when you fight him. In recent memory though, Master Gee & Voracidous from Borderlands 2. Master Gee because...well everything about that fight is poorly designed and tedious, and Voracidous because NOVA SPAM and wall of health. Also, just about all of the boss encounters from Syndicate were terrible, especially the final boss.