-The animation is WAY TO LONG! I have 5-7 time, where I thought that the part 1 ended. You cannot make animation this long! Just trust me on that.
-It's too choppy.Also there are 2-3 parts, that are really smooth for no reason. I dunno, if these parts are stolen or something but that's kinda fishy.
-50% of the stuff takes too long (Sonic being knocked back, Shadow shooting the beam, charging beams, etc.)
-The transformations are all over that place. First there's fire sonic, then dark shadow and then there's fleetaway super sonic with dark sonic combined.
-Why the dark form can perform fire attacks? (pothole [kinda])
-At some point Sonic could fly up to shadow and attack him. Normal sonic cannot fly and that's an another pothole
-There are some “gag” poses, where sonic has big eyes and stuff like that. I just doesn't fit.
-The fight in general is dull and you can expect the whole fight (Sonic and Shadow are fighting, -> Sonic gets knocked back -> Sonic transforms -> Sonic beats up Shadow -> Shadow transforms -> Shadow beats up Sonic with a beam or chaos blast. Rinse and repeat.)
-The cloud in the sky is cut off
-The Sonic world is fused with Mario's world. Just because (even't tho it's not a crossover or anything)...
-The shakes at the beginning are too strong. Look at some fights with shakes as a reference
I know you just started and everyone start is bad, so that's why I'm giving you these tips and wrote this review, so you can look at your work and fix the stuff that's bad.
You don't need to stop making animations, cuz all you have to do is improve yourself.
I hope your career will shine and show you the path.