If you are having trouble killing, you need to ask yourself:
1: Which kill moves am i using the most?
2: Which of Sonic's kill moves do I save for the kill, and which ones do i use for most of the match?
3: How's my ability to edgegaurd and pressure opponents offstage?
4: And when do i usually go for the kills
Check the percents and situations the best sonic players will even go for kill moves. You may be going for them too early, or you may not be setting them up. dont just throw them out; try to make it so your opponent has little choices to make to counter it. doing something random gets them too somtimes
go to the sonic boards and watch vids man. thats the bbest way to compare