Again!?
Another cheesy crossover!? With fighting between popular characters!?! Oh my God, that's so original!!!
Seriously, though, don't make a stupid fight between Mario, Sonic, and Megaman. There are already too many of those. Here's just some things that stuck out:
1. Some background colors from sprite sheets (like Sonic has a green-like color) were not deleted from the sprite.
2. Some characters (Megaman was most noticable from the closeup) moved when they did something (i.e. blinking). Use onion skinning to compare sprites per frame; it will guide you to keep the sprite still.
3. 16 and 32-bit sprites were mixed. Keep it on one bittage or it will look weird.
4. Some custom pose sprites were actually stolen (Mario's middle finger from Randy Solem, Mario's angry spinning from Paul TerVoorde, which I could be mistaken due to the ease of making it) and some ideas were stolen (Mario flying taken from the RotMK series and Super Mario Reloaded; the breaking up of the monkey group was a blatant rip of Super Mario Reloaded; the profile shots of the character entrance was taken from the first Mario vs. Sonic), so I suggest you give credit where credit is due especially if it's from popular Newgrounds users.
5. As noted above, it was a very unoriginal idea. Try coming up with something other than crossover battles.
Hope this helps with creating your next submission.