How does a player gain exp when he kills an NPC?

How does a player gain exp when he kills an NPC? Here are the scripts from the weapon and the leaderboard script. Thanks!

Script in workspace:

local levelup = {}

game.Players.PlayerAdded:connect(function(Player)
    local stats = Instance.new("IntValue")
    local lv = Instance.new("IntValue",stats)
    lv.Name = "Lv"
    lv.Value = 1
    local exp = Instance.new("IntValue",stats)
    exp.Name = "EXP"
    exp.Value = 0
    stats.Parent = Player
    stats.Name = "leaderstats"
    exp.Changed:connect(function()
        if exp.Value >= levelup[lv.Value]then
            exp.Value = exp.Value - levelup[lv.Value]
            lv.Value = lv.Value + 1
        end
    end)
end)

Local script in weapon:

local animation = Instance.new('Animation')
local animTrack
local debounce = false
local player = game.Players.LocalPlayer.Character

function onEquip()
   script.Parent.Handle.UnsheathSound:Play()
end

function onActivate()
    if not debounce then
    debounce = true
    if player:FindFirstChild('ForceField') then player.ForceField:Destroy() end
    animation.AnimationId = "http://www.roblox.com/asset/?id=129967390"
    animTrack = player.Humanoid:LoadAnimation(animation)
    animTrack:Play()
    script.Parent.Handle.SlashSound:Play()
    script.Parent.Handle.Script.Disabled = false
    wait(0.5)
    animTrack:Stop()
    script.Parent.Handle.Script.Disabled = true
    wait(0.5)
    debounce = false
    end
end

function onUnequip()
    animTrack:Stop()
end

script.Parent.Equipped:connect(onEquip)
script.Parent.Unequipped:connect(onUnequip)
script.Parent.Activated:connect(onActivate)

Script in weapon's handle:

local debounce = false

function onTouched(hit)
    if not debounce then
        debounce = true
        if hit.Parent.Name:sub(1,17) == "AFK Noob Lv 1 HP " then
            hit.Parent.Humanoid.Health = hit.Parent.Humanoid.Health - 25
            wait(0.5)
            debounce = false
        end
    end
end

script.Parent.Touched:connect(onTouched)