The perspective issues that stand out to me most are the little bits on the ground that don't seem to shrink into the distance as they get farther away.
Also, the two robots on the sides have both feet on virtually the same horizontal plane, which can even be seen in the right one, which has one foot lifted.
I think the nearest robot could have his back in more shadow. Also needs some longer limbs.
Please, more dead robos, and maybe some damaged ones. More detail on the ground around the guy would also be nice...ya know, make him the center of attention. Right now, I dunno where I'm looking on this piece.
It'd add some sweet eerieness if you added an army of glowing robo-eyes in the distance.
I really like the foggy look of this piece, and that robot silhouette that's holding the gun...freaky. They're frightening but cute, just as homocidal robot armies should be.