For how top tier and annoying I hear Sonic's supposed to be, I really don't face many, at least on For Glory. Interesting to hear about the others I should be looking out for here, though.
Sonic is fast, obviously, but there are a lot of openings and it's those openings you have to learn to punish. Just go to Training mode and test him out.
Is it even possible to punish Sonic? I feel like as long as he doesn't dash attack, he's safe. Maybe you can bait him? I don't know, I find going against Sonic frustrating because if the player if good enough, it seems impossible to punish him.
Edit: I usually can stop him with flamethrower, but Charizard is such a slow character, approaching him is almost impossible.
Sonic is fast, obviously, but there are a lot of openings and it's those openings you have to learn to punish. Just go to Training mode and test him out.