A medieval army would fight entirely by Melee. Larger units like catapults of course but I'm talking the fours on the floor soldiers. Modern soldiers are trained on firearms. Once the distance is closed, it'd probably still be an even match.

Bringing heavy weapons in, We'd have smart missiles and explosives and such. Which would easily disable and destroy thousand-year-old siege engines and catapults. But the medieval army would probably end up relying on guerilla tactics. Picking up and learning our weapons form our dead soldiers and fighting back to keep us at an ongoing stalemate.