It's Robo-Trixie and Mecha-Twilight fighting each other to the death, do you really need much more description than that? You do? All right, well how about this?

When an evil asteroid threatens to destroy Ponyville, Twilight steps into the save the day. Unfortunately, doing so accidentally causes Trixie to be zapped with SPACE RADIATION, which, as we all know, makes you grow into a giant robot with lasers and shit. Of course, Twilight then has to stop the monster she inadvertently created. Double of course, the only way to do that... is to become a giant robot herself.

Written because fuck it... why not?