ill tell you right now bro...
i have absolutely no idea. ive said before, that i dont even bother playing MK with sonic, its just an excercise in futility. i play lucario specifically for that matchup. but what i do is that
1. you can try to spring and fall as slow as possible so as to timeout the tornado. dont down air
2. my personal choice most of the time, run. it works for a lot of moves, but if you have the space, just run away.
3. learn to shield-shift. if your shield is fresh, you can block the whole thing, but most good MK's will try to place the tornado so as to shield stab you. so you need to learn to tilt your shield around so you can block the whole thing.
other than that, your pretty much effed, just like how you would be in this matchup anyway