I enjoyed the animation personally. Though it is lacklustre, for a first. It's nothing to be ashamed of so keep your chin up : )
A few tips about sprite animating though, when you import the sprites onto the stage and trace your bitmaps make sure you have these settings
Color Threshold: 32
Minimum area: 1 Pixels
Curve Fit: Pixels
Corner Threshold: Many Corners
I find this gives me the best results when working with Sprites, it makes them crips and very clean.
That said when they are traced you are still left with a White or Light blue area that I saw in your animation, You can't just delete the box around the sprite itself you need to look closer at each individual sprite and ensure the background it once had isn't present. Using the selection tool, clicking and deleting will suffice for this. Just double check once you're done animating the movement itself.
Secondly I see some wobble in your movie clips ( If they are not movie clips I highly recommend you animate Mario's Regular Standing stance into one rather than do frame by frame, it saves you so much time. )
To remove this, "Wobble." where your characters are softly shifting side to side or in different directions: Once you've traced your image and removed the background make it into a movie clip and then go to your time-line.
Make an empty keyframe immediately after the first. Then turn Onion skinning on. If you look slightly below the layers and where all the frames are you should see pairs of boxes. The first pair should be blue and transparent, second should be pure white, and dark blue should be last.
You want to click on the slightly transparent set, this will let you see the last frame of your animation. If you did it correctly a ghostly image of Goku should show up and he should look partially invisible.
Next import the second frame of his sprite, Trace the bitmap and then drag it approximately over top of the ghost-image of Goku, zoom in as faaar as you can go on his Feet. Left or right, it doesn't matter.
Then Make sure that the Solid Goku you have is perfectly aligned pixel by pixel on top of the ghost image of Goku. You just have to match the soles of their feet,
The Soles of their feet. Nothing else. It should animate naturally from that.
Be very scrupulous when aligning Solid Goku over Ghost Goku and make sure the soles of each foot match each other exactly to prevent wobble. This will make your animation look alot more clean.
While it's ballsy to make your own Play button for your animation I highly recommend that you use one of Newgrounds prefabricated pre-loaders until you're of a higher experience level.
Just go to Newgrounds.com/downloads and on the left hand side there should be a Flash Loaders category.
Then open it in flash and follow the instructions. This gives you a solid loading bar, Play Button and prevents the movie from repeating against the viewers will. Which would be essential if you had audio, especially music.
If it repeats and you have music the two sound clips may actually overlay and if you leave it going long enough you'll get a music clusterfuck. So just be cautious.
When you do decide to ever add sound or music be sure to right click it in your library and go to properties and reduce the Sample Rate. This will massively compress your animation at the cost of sound quality. ( Test your animation a few times by going to Control and Test Movie until you get the sound how you want by fiddling with the audios properties. )
That said, I'm fine with another Mario Bros x DBZ and I hope to see more from you in the future.
One last thing. Be sure to get sprite backgrounds only, make sure they have as little, "Noise." as possible, meaning they're as crisp and pixelly as can be.
Story doesn't matter for now so don't bother with text, just try to make the best fight you can and have fun with it okay? In the end the more you animate the more you'll get better and THEN you can include a story, or even move onto other things.
Thank you so much for reading, I hope this helped and Cheers.