The original Source? Which original source? We have Scottland, Wales, and England to draw inspiration from. Pre-Arthurian Merlin may have been a Zeus export from Rome as an old guy with a pointy hat and stick, the god Wutan (Odin) or any number of "Original Legends of King Arthur" (I'm sort of just joking with you here, I don't mean to give you a hard time).
King Arthur, and Merlin, FYI, might be one and the same person. Thus, the battle could be Zeus vs Dumbledore, Odin vs Dumbledore, or "Some kind of wizard with a stick and sword vs Dumbledore".