Is Sunshine Smiles was an elite fighter, what kind of magic attacks would she have?

I imagine her to be the type to rush into battle and stay there with tenacity. She'd probably use high-octane explosive attacks, often involving fire and the like.

Can you think of any cool moves/spells for her?

5506848 Sunny would probably use lightning in addition to fire because electricity is also explosive when used right. She would also use a more unconventional weapon type, something that fits here energetic nature and can be used in more unpredictable ways. It helps to think of her in the context of Pinkie as she would also be likely to be somewhat random and unpredictable. This would give a very surprisingly powerful and explosive fighting style that is probably going to cause some property damage in the process.

Unpredictable, enthusiastic melee fighter who overcomes her enemies quickly with an all-out attack of such a bizarre style they have no idea how to counter it.