LOL, I like how virtually every match-up query that's popped up in the last couple of weeks has had to do mainly with characters I faced in brackets at OH SNAP!.
I played Mr. 3000's Sonic in my second round of Winner's Brackets. We played on Final Destination all three matches. =) I had to make a lot of on-the-fly adjustments in this particular match-up, because I frankly had never, ever played a Sonic before in tournament in my life up until that point, much less one that is touted as the best Sonic in the nation. The only thing I knew about Sonic was that he has Diddy Kong Syndrome (i.e. a severe limitation as far as KO power and options are concerned). His solution was the same as mine: preserve your kill moves (F-smash and B-air in Sonic's case) until the appropriate percentages. The majority of his offense comprised of Spin Dashes canceled into aerials, as well as Spring Jumps into D-air cross-ups.
The major component stringing together his attacks was his Spin Dash approach. Banana peel traps on the ground are obviously ineffective, as Spin Dash allows Sonic to roll right through them. The thing that makes Spin Dash much trickier to counter besides the fact that it can close an outrageous amount of distance in the breath of second are the Spin Dash-canceling variations (of which I didn't know existed until I played 3K). Retreating cancels, cancels that pop Sonic up vertically, etc. are all very hard to predict and readjust to once you have taken the bait to attack him while he is charging his dash.
The thing that I started doing mid-match in the first match was to not even bother approaching with glide-tossing, but to force his approach from afar with peanuts. I would then bait his Spin Dash, and then I would punish accordingly with banana peels (as well as reverse single dribbling if I needed to adjust my spacing). Banana peels will knock him out of Spin Dash state if they are thrown directly at him. If you can get this far and knock Sonic out of positional advantage, you can apply constant pressure and pretty much tack on endless damage, especially if the Sonic player begins to play defensively (which is always a bad choice for that person because Sonic's out-of-shield options are really quite awful).
If you for some reason aren't able to react fast enough to counter Spin Dash as soon as you see it coming, try your best to just power-shield it instead and retaliate immediately with a glide-toss cross-up before he leaves your rear range. Sonic has a slight slide to his tripping animation; following up against him isn't as bad as Luigi or even Yoshi, but Sonic recovers from tripped state fairly quickly, and I was caught off guard a few times at how that slight slide made all the difference between me ****** 3K with a dash attack combo and, well, me not comboing him at all.
I was able to rack up damage fine by playing defensively. The problem by the time the final match rolled around was that I wasn't able to kill him as quickly as I wanted because he started playing defensive hit 'n run, as well. In the event of this, it is imperative that you never use your F- or D-smashes until you are positive they will kill.