against Sonic dair camping is really effective. Make sure there's sound on the TV so that when he's charging his spindash bull**** the moment you hear him release you can air jump dair, and that tends to cover his options.
if you're near a ledge, fall off uair is like really good, he can't do anything to stop it (like say you're right next to the ledge, and you just press in the direction so you're off the stage and instantly uair) if he's in a spindash anyways.
Watch out for his fsmash, it's like... super disjointed. If you're offstage, you really have to be careful not to throw anything out, 'cause it like... just beats ****.
Dair is like really good against Sonic. Tornado isn't that good against a Sonic that knows how to stop it, a lot of the time tornado is what they wait for, because he can just shield it, and then punish you afterwards no matter where you go because Sonic's cheap.
If you get up thrown, and you don't DI it, and you see him spring, usually you can just throw out a dair on reaction, and he can't really do anything about it. It's not like he can change his momentum to bait the dair and then attack you afterwards, if he's springed, his vertical movement is now completely predictable, apart from ability to fastfall/dair which isn't relevant in this position. So if you see him spring, usually you can just throw out dair (on reaction to seeing spring, don't just throw it out without seeing him spring, because if you do that he can mix up his timing and predict you, because you're working off prediction, not reaction).
Idk maybe Sonic has something he can do against just dairing him when he comes at you after Uthrow, but I haven't seen it, and it's worked for me in that position against every Sonic lol
You can juggle him pretty well, tornado is good here for juggling if he's already springed. Like if he's springed and he's just cartwheeling down to the ground, and you tornado, he can only really dair to get away, air dodge, or fastfall (and stuff after fastfall if he wants). Well if you space it like ****tily he could drop into your nado and do **** but try and make it so he's not able to do that, so that you don't need to worry about it.
If he takes you to yoshi's, and he charged up his side B on the ledge because that's what he'll do, tornadoing is really safe. If he's fully charged his side B, his options are to do his spinning jump thing (where he's still in ball form and just jumps up) which can evade tornado, but with that platform in the middle he has no real way of punish it if you retreat well. Or he can continue his spindash. Moooost of the time, tornado clanks with his spindash, and that causes his invincibility to go away, but your tornado still has hitboxes, and then he gets swallowed up by it. But sometimes tornado gets cancelled out and stuff. Idk it's pretty inconsistent and I don't know what causes it, in practicality it's kinda random, so the majority of the time your tornado should win out. But I guess keep in mind that dumb Brawl stuff can happen here lol
If he hasn't charged up his side B completely, like if he's just started the spindash charge thing, he can shield cancel it, and if he does that, and then you tornado at him, he has a legitimate shot of punish you, so don't throw out preemptive tornados unless you're pretty sure they'll hit, but if you're like at a stalemate or something for a moment, or you just see that he's waiting for you to do something and he's already fully charged his side B, you can just throw out tornado and it's almost completely safe.
On a normal stage, if his spindash hits your shield, and he jumps out of it, a lot of the time you can just up air him out of shield, or up B him if you guess which direction he goes (like a standard up B if he doesn't cross you up, but if they hit your shield with spindash, I think they almost always cross up, so a turnaround up B can usually beat whatever his option is after a spindash).
I think he can hit your shield with a spindash, and immediately spring out of it, and be unpunishable at first, like if you try something out of shield and he picks that option, he'll never take damage or something lol. But then he's also way high in the air after a spring, and you can kinda just throw out a tornado in the direction he's facing (to cover his auto-cancel down air option).
I'm not 100% sure on some of the out of shield stuff, I always feel kind of inconsistent here, because sometimes I'm fast, and Sonics hit my shield and then I just punish them with a juggle or something, but other times maybe I mess up a buffered uair and I get hit by a spring, or a bair or something. Like if I'm playing on point, and precise, I feel like Sonic's unsafe on shield, but I think you need to be kind of fast to do that, and I'm really inconsistent in my in-game control in general haha
Sonic's really tough, because they all know how to play the MK matchup, and if you go on autopilot, you're gonna get *****, because they're all trained to punish everything you whiff. Sonic can bait you really hard.
Doooon't go for shorthop fairs, he can just hit you after them and they don't protect you lol. Fullhop fair can be pretty good because it's often a really awkward angle for Sonic to punish, and by the time he's caught on to the way he needs to punish it, you often have enough time to rising dair him.
If you're juggling him, and you're gonna tornado, and you're just a tiny bit unsure that he's able to air dodge through it, don't go for it, just continue with your uair traps. If your reaction time is quick you can uair his dair. Don't just go for shuttle loops that have a good chance of missing, but also don't forget that getting an early kill is HUGE in this matchup, especially. Sonic does have trouble killing, and if you just don't get hit by his spindash, and you don't walk into his Fsmash (it's not toooo hard, but it is much harder than you would think, it's a lot more disjointed than it looks lol. If you're at a kill percent and you're unsure, don't challenge it because it probably has more range than you expect haha. Be careful about that **** when you're on the ledge, too. If he's charging his Fsmashes at the ledge and aiming them down, see if you can wait until you see him release the Fsmash, and then fair him afterwards or make your move then, but not DURING his fsmash charge, because it beats a lot more than you'd expect, a lot more consistently than you'd expect), then he's not gonna get kills particularly easily.
and if you die to something unexpected early, make sure to shrug it off, especially against Sonic. He can hold a lead against MK, but not nearly as well as you can hold a lead against him, as long as the timer isn't too much on his side, you're NEVER out of this matchup.
In general, juggling is really helpful because he doesn't have many options after spring (keep that in mind and figure out for yourself the best way that you personally can take advantage of that), try not to get hit by anything especially dumb because living a long time is one of your best advantages in this matchup, and don't throw stuff out/on his shield because he can punish like everything on whiff other than a rising dair. Oh and dair camping is good if you do it right.
idk this is all I could think of about this matchup, and I'm probably wrong on a lot of it, but this has been my experience with it from playing X, and some socal sonics and some vegas sonic recently. I hope it helps you, I'm pretty done with the game so I figured I might as well make what's probably my last post on the Brawl boards a hopefully constructive one for somebody c: