Edit:
I think the most important change I made is timing, I made the anticipation frames take longer and the slash frame shorter. This makes it feel more elastic, a long wind up and a short execution makes it feel stronger. Of course if you want it to be a weaker attack then you have to weaken the anticipation.

Another thing i tried to do is have some restoration frames, so when they are done with the attack, they have to get back to their normal position.  I don't think they are really mandatory, but I think it would improve your animation anyway.

Edit:

These are all some suggestions to make it better. Firstly, since you are new to pixel art, I take it you are also new to art, so your shading and colours arent that good yet. You can see that I added some contrast and shading to suggest form. For colours i'd say, experiment as much as possible until you get the right colour. Shading takes a lot of practice as well. Both of these are art fundamentals and there are tons of tutorials online about them.

Now for some more pixel art related comments. You use black lines everywhere, for the outline this is fine, tho a dark grey or purple looks really nice as well. For the "inlines",lines in the sprites, i'd recommend to use more soft colours. As you can see thats what I did for the arms. Secondly, you use some 1 pixel clusters, a cluster is basically a shape of pixels of 1 colours. so a 1 pixel cluster is just a single pixel of 1 colour. These look pretty ugly so try to avoid those. Only for eyes i'd say you should use those, or if there is no other option(which there usually is). So for example those scattered orange pixels above the bomb, or the highlight above his eye, those can all be fixed by either removing them or making them bigger clusters.

So then i have a character comment. The bear seems pretty non-reactionary to the bomb. We don't know how he thinks about it. He has an exclamation sign next to him, but his face doesnt seem surprised at all. The bear could also look like he is planning to do something with the bomb then he could look like this:

Either way its good to show some character. the bear looks really lovely and hes a cool persona, so try to give him some personality.

@eishiya, the giveaway is purely to get attention. And since we want as many stayers as possible, I think a pixel art commission would create more stayers than a video game or whatever giveaway. Since pixel art is what the site is for.

Could the giveaways be in a from of a free commission? Like a small sprite by one of you really good pixel artists. Then you don't have to be annoyed with shipping costs and stuff, also you only attract people who are interested in pixel art.