TFS 1.X+ Perfect paralyze rune

[a Shadou]

I've made script for paralyze rune but i have one problem.
This script requires target to calculate how much speed it has to decrease.
Problem is that you can't throw rune without targeting someone. Example: you won't be able to use rune through battle list.
Does anyone know better way to find rune target that is not targeted?
I was considering doing it through events like

Creature:onTargetCombat(target)

function onCastSpell(creature, var)
local target = creature:getTarget()
    if not target then
        creature:sendCancelMessage("You need to target creature first.")
        creature:getPosition():sendMagicEffect(CONST_ME_POFF)
        return false
    end

local combat = Combat()
    combat:setParameter(COMBAT_PARAM_EFFECT, CONST_ME_MAGIC_RED)

local condition = Condition(CONDITION_PARALYZE)
    condition:setParameter(CONDITION_PARAM_TICKS, 20000)
    condition:setFormula(0, -(target:getSpeed()) + 80, 0, -(target:getSpeed()) + 80)
    combat:addCondition(condition)

    if not combat:execute(creature, var) then
        return false
    end
    creature:getPosition():sendMagicEffect(CONST_ME_MAGIC_GREEN)
    return true
end