okay.
I will try to be as fair as possible here.
problems-
First, you need sound effects.
second, there is nothing to distinguish this from all the other hundreds of sonic vs shadow flashes here. Try to do something unique.
third, There should be no white backgrounds on the characters. I can't stand to see that.
good sides
First, I liked the song you picked out for this fight. It seemed to fit the fight well.
Second, I liked the fact there was some story, even if it was a little underdeveloped. Try to make a little more happen in the story.
third, i liked the chose a winner. I can tell you like sonic more, because you didn't give shadow his super form. Next time, try a few extra endings to give it an extra kick.
fourth, I did like how the background was destroyed alittle during the fight. You should have had a real background rather than a blue background.
So, try to improve a bit. I hope you get a little helpful advice out of this. This is the fairest I think I have ever been on something this overdone before. So, you should be feeling lucky.