I love hypotheticals like this — they make me giddy. If I had to pick a single most important rule, it’s that context is king. Put 'Harry Potter' and 'Percy Jackson' in a hallway with a few suits of armor and Harry’s got a lot of advantages: precise wandwork, a repertoire of defensive and controlling spells (Protego, Stupefy, Petrificus!), and a history of outsmarting foes through planning and clever uses of magic. Harry’s experience with things like Horcruxes, the Resurrection Stone, and the Elder Wand (if you want to go full Hallows) gives him toolkit options that are wildly versatile. He’s patient, resourceful, and his spells can be instantaneous—disarm, bind, immobilize. That matters in a duel.
Now shift that scene to the open sea or even a riverbank and the balance tips hard. Percy’s whole deal is elemental control: water isn’t just a power, it’s his lifeblood. In water he heals, grows stronger, breathes, and can manipulate tides and currents at scale. His swordplay with Riptide (Anaklusmos) is brutal and precise; he’s trained as a fighter and is used to direct, lethal combat against huge monsters and gods. Percy also has the durable, battlefield-tested instincts of someone who’s constantly facing beings that don’t follow human rules.
So who wins? I’d say it’s situational. In a neutral arena with little water, Harry’s magic and crafty thinking could win the day. In or near water, Percy becomes a force of nature that’s extremely hard to counter. Personally, I love that neither outcome feels boring — both are heroic in different ways, and I’d happily watch a rematch under different conditions.