(little note: I will only be focusing on the things that needs to be improved. I know, it sounds mean, but I simply do not think it will be productive lol)
ok, first things first:
No, seriously do not use brush for these sort effects. It does not look nice, especially if it is not with high smoothing.
By using shapes and a filter, you can get something smooth like the one on the right. See how the one on the left is jagged and edgy compared to the other one. You can easily do this by getting a circle shape (make sure you hold SHIFT so it is a perfect circle) and make it have a radial gradient, both being the same colour...white (furthermore, make sure the fill on the right side has an alpha of 0%, like this one)
From there, you can animate it by making it thinner but longer frame by frame.
NOTE: IT DOES NOT HAVE TO BE 4 FRAMES. IT IS UP TO YOU HOW LONG YOU WANT IT TO BE. However, I would recommend that you make it as long as your original slashes so it doesnt mess up your timing of your part.
Another issue would have to be from this point where you appear out of the blue (nice scarf flow btw)
When the bodies fall, they will still have blood running out of their bodies. Try to add that by tweening a blood pool getting bigger. Obviously, dont make it appear immediately, smooth it out B]
When you quickly zoom away like this, it would be really cool if you could add some big dust cloud behind you to emphasise the speed you are going at. Maybe add some camera shakes to make it look powerful! Also, at the point where you move the people away, I think it would have been better if you added a slow-to-fast easing transition to make it look like he is going ultra fast.
You are supposed to go sanic speed, remember? Add some wind effects to show that! There isnt much happening on that screen. You could add some people zooming past, too!
When the enemies appear, they're standing still. They are supposed to be running lol. Unless they saw your RHG from miles away and had that korean reaction time to stop and prepare to punch him in the face, then I don't think they would stop XD. However, there is a great concern at this certain part of the anim. Everything is a bit slow. The enemies should be traveling almost 1/2 of the stage per frame! They definitely shouldn't be reaching half of the stage in about 4 frames lol. adding blurs on them is not a bad idea, either. The audience's focus is not on the enemies, but on your RHG, so making them disappar quickly will not be a concern. Additionally, because your character isn't really running, you should add some dust effect as his feet are dragging on the floor, covering a lot of distance quickly. Your combat is pretty good, tho.
Your character does a kick and fires a fire projectile at the enemies, but there are some issues with what you did here. First of all, your fire 1 frame long. Dude, that is not good! it should be an animated loop, as it will make it look a LOT more appealing. Also, as it is the centre of attention for the viewer, I suggest that it stays in place. If the fire moves arounds like that, it does not look too good, UNLESS you actually animate it going around them. In that case, it can look very good! Another thing is that the way you make the enemies go past the flame looked very awkward. Making every single of them have the exact same pose makes your anim lose quality, and makes you look kinda lazy :/. If you look at my version of the image, I put everyone at different places to show perspective, making it look visually appealing (although everyone is in the same pose, if I were to animate this, they would have different poses each frame (similar to how you made your enemies run in the following image, which I will be talking about now).
This part is not as bad, but like before, you can move them away from each other and add some perspective, like my one. Having them in a line like that makes them look like the TMNT lol. They are a barbaric army that have no real game plan, so they're basically all crazy XD (I could have told you this before. My bad).
Finally, as the picture suggest, if that guy got absolutely obliterated by the flames, ass a flame burt effect and some nice thick blood explosion!. Also, which the sound effect of the explosion, I think it would be better to have that play the moment the screen flashes white, as that is where the the fire hits them all.
In conclusion, your parts have quite of bit of things to fix. Feel free to VM or PM me about anything you need further help on :)