Prep is overrated against a guy who can move at the speed of light and shoot lasers hotter than the sun out of his eyes.
Now that isn't to say that I cannot think of a plausible situation in a comic where, with enough time to prepare, a guy like Tony Stark could find a way to kill Superman. However, I think that would entirely depend on Superman not fighting to his fullest or not taking advantage of all of his powers (this isn't a stretch as he has stated before he takes it easy on his opponents). But in an arena match with both characters fighting to their best, I just can't see Iron Man lasting more than a second no matter how much prep time he has.