You can certainly slash away at his huge health bar using your tiny weapons all you want, however, the designers wanted players to Elden Ring Runes use a specific sword in the arena to strike his cranium, which is looming in the distance, with wind-powered magic instead. This turns extremely difficult fight an easy task and is appropriately thematically.
The battle of Radahn in Elden Ring is also all about making use of the resources available to you. Remember when I mentioned that a number of warriors gathered in Redmane Castle for the festival? That means that you'll be able to summon not one, but not two but six NPCs to assist you in destroying Radahn. It's not like they can make the battle as simple as smashing Yhorm to pieces using Storm Ruler, but having his huge attacks focused on someone else as you engage in hit-and-run-style strafing attack against your horse much more effective than having him in your midst all the time. Even better, when your friends die--and trust me when I say that they'll die often--you can summon them over and over. It is the rule.
You can summon Patches to join the Radahn fight , too, but don't count on him to stay in the fight.
In any event the fireworks will happen after Radahn defeats him. As Sellen stated, his death will ensure that the stars will be able to move again. They must be wanting to move their feet, however, as they begin moving across the sky en masse and filling the sky with Elden Ring Runes for sale luminous illumination as they resume their normal cycles.