How would I make an npc damage the person that attacked them?

You can add a 'tag' in the NPC that holds the player's name, then using the 'ChildAdded' function you can search for the player and target it. In your weapon:

local Tag = Instance.new("StringValue")
Tag.Name = "Damaged"
Tag.Value = Whatevertheplayeris.Name
Tag.Parent = WhatevertheNPCis
spawn(function()
	wait(10) -- Expire after this time, so the NPC does not target the Player forever.
	Tag:Destroy()
end)

In the NPC searching script:

WhatevertheNPCis.ChildAdded:Connect(function(child)
	if child:IsA("StringValue") and child.Name == "Damaged" then
		local Player = workspace:FindFirstChild(child.Value)
		if Player then
			-- Do whatever you do
		end
	end
end)