Hey there!  What you have isn't too shabby.  Here is an edit I've done to help show what you could do.  (hopefully this looks alright.  My monitor is a bit screwy with colors)



What I did
1. The most important thing I did was change the hue for the shadows. 
2. Shifted your colors around so they have more contrast (greater difference between the colors.) Low contrast being less difference.
3. Changed her pose so she has more balance.  I'm not sure if that was a good choice.  Maybe you have her posing like that so it matches the clothes.  I don't know if they have been made yet.  Likewise that is the reason I didn't change her foot.
4. I attempted to shade her in the same sort of style you have going on.  Wasn't able to finish though.

Anyway hope this helps somehow.

Also I liked the pose you had before.  That pose is more interesting.  I can see why your client wanted a pose like this though.  Now the characters are able to hold objects and you can see the clothing better from the front.

I'll agree with jams here only if the robot is supposed to be massive.  If so then yes you need to have the a large amount of slow build and then very even spacing with a lot of frames.  

Take a look at this http://brendanbody.blogspot.com/2010/04/weight-problem.html

This should help.  It's helped me out loads.

If it's not supposed to be massive then you can get away with quick speedy movements.

However if it is massive and still moves quickly only the comparison of it's size to others will inform how big it is.  Also if it's on it's own in say a close up action sequence it won't feel massive because there will be nothing to compare it to.

I suppose you can have a happy medium.  I think.

Oh something else you can try.  If the robot moves fast; other smaller things could move faster.  That might work.

Something I forgot to ask.  Is that phallic part telescopic?  If not then how can it punch the ground like that?

I don't know what it is but this specific area...is amazingly attractive to me.  Who knew lighting could be so hot?

Nothing to contribute (yet) other than you somehow made blue and purple work nicely together 

This is how i would do it.  I've seen this type of scene before in anime.  It's normally at either before the commercial break or at the end of the show.  A sort of omnious bit of either color pencil or water color work.  Anyway what they normally do in this scenario is position the bad guy as this sort of omniscient god like creature.  Positioned above the main characters and larger than life.  Here is an edit I cobbled together.

 It's a bit messy because I don't have my tablet with me so I can't do any sort of skillful edit.  Anyway what I did was made Kaiba bigger and higher in the sky.  As well as lowering the contrast and slightly desaturating him.  Basically blending Kaiba in with the cooler color of the sky.  Also the Blue Eyes White Dragon overlaps with Kaiba that way it's more of an obvious separation from the background.  Also it too is bigger and a bit more saturated.  

Oh and it would be cool if Kaiba was drawn sort of curving inward like he is enveloping the heroes.

Other than that it's look great!  I look forward to the finished piece.

StarRaven I just wanted to commend you on a job well done for that run.  it's effectively 4 frames visually but only 2 frames in actuality.  Wow really impressive stuff!

PixelPileDriver I've never seen such fluidity in such a small sprite.  It's a little jarring.  Like if a whole game was made with that many frames with such small characters it would be quite...unique.