Make sure you are always predicting Sonic's attacks and movements. Nothing is worse than being constantly being beat up by a fast character due to them tricking or faking you out constantly. With Jigglypuff, stay stationary, because his attacks usually leave him vulnerable to fast characters like Sonic. When Sonic comes rushing toward you, simply counter with a neutral-air or forward-air, knocking him back. Since it's difficult to keep Sonic in a combo in the air, just try to rack up damage consistently by blocking his attacks, striking, retreating, blocking, striking, retreating. Also, although this may not be that helpful, sometimes floating offstage will cause Sonic to jump offstage to hit you, but you can retaliate as Jiggly's aerial game is much stronger than Sonic's if used correctly. Never risk using Sing or Rest, as Sonic most like avoid those.