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

Simply use a cooldown or so called debounce into it. Here's an example:

--Script inside a part for example

local cooldown = false
script.Parent.Touched:connect(function(hit)
if not cooldown and hit.Parent:FindFirstChild("Humanoid")  then 
--If "cooldown" is "false" and it's a player then..
hit.Parent.Humanoid:TakeDamage(10)
cooldown = true
wait(5) --Wait 5 seconds before it deals damage again
cooldown = false --Since "cooldown" is "false" again, the barrier will now deal damage again!
end
end)