For killing, just be patient, it gets annoying when you die at like 90% and can't kill reliably till 130%, but Sonic is just about pressure and evasion. One thing that I've really been trying to do with my game is be way more aggressive edgeguarding, since Sonic can basically recover from anywhere, and his fair, bair, and sex kick all can be useful offstage (fair and sex kick are better than bair offstage, which can be DI'd upwards and actually help the person recover), but if that doesn't work, dsmash is also a killing move, moreso than fsmash, but is also harder to get in. If you can predict your opponent well, perhaps baiting an attack by running in and shielding, then it's great for KOs. Killing off the top isn't reliable, but you can still come in fairly quickly with a spring -> aerial and kill off the top, or if they dodge, you can dair and return to the stage before them, taking advantage of the fact that if you dair from high enough (the height that the spring sends you), Sonic won't have any landing lag.
As for a semi-reliable way to get in a potential KO move at over 100%, do overB or downB into a sex kick. It may seem like it's a bad choice since they have quite a bit of startup, you can shield cancel the overB if you run in and start it close, tricking them, either shielding their counter-attack or grabbing them after a spotdodge, or do either from further away and punish something that they had done.
As for living longer, there's not much to do against being killed vertically, but just try to be more evasive and not give up a chance to get hit. This doesn't mean camping, you'll be in constant motion most of the time, but really just using all of Sonic's feints and mixups as a way of evasion. If your opponent can't guess what you're going to do next, it's easier to read them and choose the option that's most advantageous for you.
Hopefully this rambling helped, Sonic's a character that has to use the player's mental abilities more than his abilities, since he really isn't amazing as a character. The thing I'd really stress though is aggressively going out to edgeguard the person, even if you just jump out and make them return a little lower, you can use that to hit them with a spring, which might push them out of recovery range, or might make them return higher the next time, which you can use to actually hit them with an attack of your own. Sonic has a lot of options both on and off the stage, but all are mediocre, so it's up to you to make the best uses of them.
