Champions of Equestria

I'm rating this after the wonderful piece you did w/AJ.
I like this piece and can tell you have put much thought and effort into it. You use your space very well, RD's pose: moving right while looking left, adds interest and motion and holds the viewers attention.
I like the look of the battle armor, it appears functional and the streamlined look fits the wearer.
However, I think your handling of RD's mane and tail needs to be addressed. The individual hairs drawn under her helmet near her eyes look great, but the flowing mane and tail are a bit too much. I think you would gain by suggesting hair volume rather than drawing more hair lines.
Her expression doesn't seem to fit a mid-battle scene. You suggest a fierce glare, but it doesn't go far enough. without the context, she could just be squinting or trying to view at a distance. Try suggesting a bit more downturn near the nose, or upper brow lines to give a better "hooded" expression. Also, RD's mouth doesn't quite sell the fierce emotion. I can see that you did add a subtle grimace, and show tightness with the lower lip-line, but I think you could benefit from curling the upper lip more, and maybe showing some teeth. I think you could do it without modifying the cheek-guard.
About Horse anatomy... (I hope this isn't taken wrong, I know this is a cartoon) RD's Forelimbs are so wide that she looks like human football player. Horses have their forelimbs jointed under the body not off on shoulders like humans. if she were standing, I wouldn't comment, but as you have her posed like a running equine, I find the positioning to be a distraction. If no-one else mentions it, I may just have to get off my "realistic" high-horse (high-pony?) and deal with it.
As for the background, I like the parts under her wings, Arrows, spears, and rocks, simple and yet recognizable. However, Twilight looks stiff and block-like. She is too far away to show much emotion or action. The tree and waterfall in my opinion are too much detail for background and compete for the foreground lending to a flatter feel, when I think you want depth. The moon in the background is done very well, and looks good.
I think I know the effect you are trying to achieve, and believe it would have been better represented by having Twilight much closer to RD. She is too much in the background to really be part of the action or show emotion. Your scene may very well be more accurate in terms of how a magic user might support the infantry, but it doesn't help your art in terms of composition, feel, emotion, and overall effect.
I know I am in opposition to others on the level of detail in the background. I hope you are not frustrated by the contradiction and can experiment on how detail and value intensity affect perceived depth. I also hope my comments are helpful and not discouraging. You have talent, imagination, and a good eye for composition. Your subject is detailed heavily (markings on the armour, sweat suggested on RD's face) without being over-done. Well done.