Seems solid to me. Nice job!

On a personal note, I'm not a big fan of blood in brickfilms, but it's your film. The effect works well, but the minifig doesn't have an entry wound.

"Nothing goes down 'less I'm involved. No nuggets. No onion rings. No nothin'. A cheeseburger gets sold in the park, I want in! You got fat while we starved on the streets...now it's my turn!" -Harley Morenstein

Very nice job on the masking. Animationwise it feels a bit choppy, but I think that's mostly because masking can make it hard to keep the animation smooth. I'd love to see a "before" image with the masking rig still in there (just out of curiosity).

Cant quite make out what that rig is.

I usually just get a chop stick with tack on each end.  Secure one end to the table and the other end is attached behind the fig.  You have complete control and movement that way and the masking is not much harder.

That said, youve done well.  I dont like blood either but that wasnt the question. 

Its a very well done attempt here! I still can't get my masking right haha!

The blood itself looks weird because it seems more like it was merely pasted onto the film rather than actually being there.  It also sort of evaporates into nothing in a split second as if it's super fast titan blood or something rather than splattering onto the set or leaving a mark on the figure's chest.
Now flowers make some great in-shot entry wounds.

The walking also looks rather weird because it looks as though you've neglected to move the arms during the end of each swing.  In walking animation, the arms must be in constant motion, easing in and out.

The knock-back itself, however, is quite nice, which is most important I suppose since it's the focus of this shot.
The only small problem with it though is that you masked out some of the shadow under the figure.  Of course, this is only really noticeable unless you pause the frame.
A good way to avoid this is when animating, support the figure from the side rather than from underneath, thus the original shadow may be preserved whilst still masking out the support and it's shadow.  There are some shots where it might be particularly noticeable.

Here's an example from an as of yet uncompleted film where Batman punches a thug and he flies really far back: