Talk to Asgore until the fight starts
Choose Fight to begin the battle
Use the Broken Key if you have it? No, it is not needed
Keep your HP high with items like Butterscotch Pie, Cinnamon Bun, Instant Noodles, and Legendary Hero
Learn his attack patterns and dodge carefully
Use healing items after strong attacks
Do not use Mercy; Asgore cannot be spared in this fight
Reduce his HP with repeated Fight actions
Survive until his HP reaches zero
After the fight, continue the story normally