Yet in vain a paynim foe
Armed with fate the mighty blow:
Means that, even though the opposing fighter knocked him down and could have killed him, Arthur was saved because he was hidden by the Elfin Queen.
"... the son of Uther, named Pendragon, a title given to an elective sovereign, mount over the many kings of Britain."
Mount over means to 'rise above'. He was to rule over the other kings.