How can i make an animation randomizer?

You could use math.random(1,4), this will select a number from 1 to 4 and then detect wich one is selected and use that animation (it printed 3 so it will activate your third animation)

slash = 0

repeat -- change this to when they use the lightsaber
    wait()
    slash = math.random(1,4)
    if slash == 1 then
        print("1")
        --animation1
    elseif slash == 2 then
        print("2")
        --animation2
    elseif slash == 3 then
        print("3")
        --animation3
    elseif slash == 4 then
        print("4")
        --animation4
    end
until false

Should be pretty easy, I don't know how your lightsaber works so the animations you have to execute yourself as well as the trigger.