Maybe you didn't copy his animation on purpose but you definitely took some elements subconsciously (the worst of them is rotating him for 180deg, it maybe work for his character but not for yours).
I sort of acted it out myself in front of a mirror and animated that.
Maybe you are not very good actor. 
I agree that he should shift his torso in order to show force of impact but rotating his hips and legs is unnecessary.
This is no mere thwack. This is a full on might of all your body overhead swing.
Yes, I probably did subconciously take inspiration. But also, you can get so much more power into a swing by putting your full bodyweight into it following through, taking a step etc. And when I tried that, I found that it was like that. That is my reasoning, and I am being
slightly defensive but that's just me.
I had to transfer it to gimp to change the contrast so it's a bit much of an effort.
That's an unnecessary hassle! Although it could be useful for early quick tests, you adjust contrast by working on your palette.
About this and shading and outlines, you should make sure to regularly test sprites against the bg for readability, before you commit to an option that might end up not working. Shaded sprites might appear very differently in context. No need to make perfect shading for this at this point.
I don't quite see hwat you mean by working on my pallette. I know what a pallette i etc, but not what you mean. Sorry.
I'll do some basic, one layer shading, and perhaps another sprite that doesn't look quite so like rosiers, maybe a smaller backhanded swing.
