The Barrier does damage, but it insta kills even if i set the damage to 10??

This could also be because it is hitting multiple parts of the game character, as a typical R15 character has 16 parts, it essentially does 160 damage, you can prevent this with a table of things it already hit, like StickMasterLuke did in his Mad Bloxxer tutorials.

local lastclick = tick()
local Tool = script.Parent
local hitHumanoids = {}

Tool.RemoteEvent.OnServerEvent:Connect(function(Player)
	local Character = Player.Character
	local hum = Character.Humanoid
	local Barrier = Instance.new("Part")
	Barrier.BrickColor = BrickColor.new("Lime green")
	Barrier.Material = "Neon"
	Barrier.Transparency = 0.25
	Barrier.Anchored = false
	Barrier.Size = Vector3.new(8,13,1)
	Barrier.Shape = Enum.PartType.Block
	Barrier.Locked = true
	Barrier.CanCollide = false
	Barrier.CFrame = Character.HumanoidRootPart.CFrame
	Barrier.Parent = Character
	local bv = Instance.new("BodyVelocity")
	 bv.MaxForce = Vector3.new(math.huge,math.huge,math.huge)
	bv.Velocity = Character.HumanoidRootPart.CFrame.lookVector * 25
	bv.Parent = Barrier

	Barrier.Touched:Connect(function(hit)
		if hit and hit.Parent and game.Players:GetPlayerFromCharacter(hit.Parent)  and not hit:isDescendantOf(Character) then
			local targethum = hit.Parent:FindFirstChild("Humanoid")
			if not hitHumanoids[targethum] then
				targethum:TakeDamage(10)
				hitHumanoids[targethum] = true --can be anything besides false or nil 
			end
		end
	end)
end)

Tool.Activated:Connect(function()
	hitHumanoids = {} --resets hitHumanoids when the tool is clicked
end)

This essentially makes a table of the humanoids hit/damaged, and checks if a humanoid that has been hit had been hit before, then if it didnt, it gets damaged and added to the table that is hitHumanoids