I was making a projectile attack but I don't know which method I should use to damage the dummy.
You can do this by either: Using the TakeDamage function on the dummy's humanoid:
game.Workspace.Dummy.Humanoid:TakeDamage(amount)
or, simply subtract from their health.
game.Workspace.Dummy.Humanoid.Health = game.Workspace.Dummy.Humanoid.Health - amount
I would recommend TakeDamage. Hope I helped.