The Kingsguard could absolutely lose this. Maegor was essentially a war loving prodigy, kicking the crap out of men in melees when he was 12/13 and his skills were deemed as being unmatched at the time. He was also a big ass man, strong like a bull. And he carried Blackfyre.
Aemon was also a fantastic warrior, deemed worthy of wielding Dark Sister, becoming Lord Commander of the Kingsguard. And noted for various deeds. Daemon Blackfyre's talent made folks believe that he could have reached the Dragonknight's prowess, Aemon was the real deal. He is one of the most legendary Kingsguards.
I don't know, really hard match. For me to bet on the Kingsguard Pride, their sum of ability would really have to be > the Targaryens, given that Barristan is fighting with a normal blade. I'm not sure if it is.