robzo100 on 19 February 2011
I'll say Super Mario World 2: Yoshi's Island. All of a sudden a traditional 2d boss battle turns into shooting eggs at a giant Baby Bowser that Kills you by taking up the whole screen with his ginormous body.
Also, I second the other Earthbound fan who mentioned Giygas. A Boss that has no physical form - just an abstract and disoriented background of red and black
