Early game · first 8 minutes
Place Radiant Daggers where Rat King stops to shoot. Move between casts so Scrap Grenade cannot keep detonating under you.
Celeste vs Rat King
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Mid game · Walkers
Use Dazzling Trick when Rat King gets close and starts shooting. If he breaks it nearby, the blast can silence him. Follow with Light Eater and gunfire once Royal Pestments ends.
Late game · Patron
Aim Shining Wonder at enemies gathered around the banner so it can bounce between them. Follow with Radiant Daggers while they are still close together, staying far enough away that Rat King cannot hit you by slamming his banner down.