As the MLP show went on the villains gradually got stronger and stronger before peaking with Tirek. Starlight Glimmer was a good successor though and most of the villains after that were pretty strong too. That stopped with Cozy though. She’s easily the weakest villain of them all and I have to say that there’s no way she would get the drop on Starlight. In a proper fight I believe Starlight Glimmer would take the win easily. A quick energy blast spell would certainly do the trick and Cozy doesn’t have any real way of defending herself from an attack like that. Starlight Glimmer wins.