Twinkle Snow boss in SAdva3. You try to climb up some platforms and drop them on Eggman's head. Climb too slow, you die. Try to climb too fast, you hit some sort of ceiling and maybe die as a result. Overshoot a jump, you probably die. Run off the platform without getting a jump off, you definitely die. Oh, and watch out for the spike ball when it comes. The one redeeming aspect of this fight is that it's entirely non-random. But there's not much room for error.
I was pretty happy to finish a run sub-50. Then I decided that I'd watch (most of) mike89's 100% run of the game on SDA again. When he did the Twinkle Snow boss, I saw that my strat was actually pretty weak. I had the spike ball after hits 4 and 7. I think he managed to get only one spike ball attack, which saved about five seconds; this, in the same run as finding all the lost Chao, collecting all seven Chaos Emeralds, and beating Nonaggression! All the worse for my opinion of my own time, to see video footage of how I could've done it better. I'm not coming back here yet, too fresh in my mind.