So first off, runs are friggin hard, you can look at my ongoing thread if you get discouraged. Iv been on that same animation for like a week now. That being said, The biggest issues Im seeing are 1 inconsistent limb sizes, 2 the arm comes up to high in the forward motion, when it bends the elbow. 3. Timing, thats easy, just slow it down a little, annndd lastly, you dont have a solid frame for the legs crossing over, so it just looks like he's kicking forwards and backwards at the same time, over and over again, without switching legs. I get that he is, because the shading changes slightly and the crotch moves back and fourth, but I can only tell thats whats going on because I made almost the exact same mistake when I started on mine.
Oh, this is unrelated, but on the sprite of the enemy with the mega buster, it looks like he's punching himself in the face every time he shoots... It does look kinda like recoil, just like the recoil is forcing the gun to hit him in the head over and over again.