Lua how to? - Spell, effect only on hit

[Szafi]

TFS 0.4

local combat = createCombatObject()
setCombatParam(combat, COMBAT_PARAM_TYPE, COMBAT_FIREDAMAGE)
setCombatParam(combat, COMBAT_PARAM_EFFECT, 6)
setCombatParam(combat, COMBAT_PARAM_USECHARGES, true)

local area = createCombatArea{
{1, 1, 1},
{1, 2, 1},
{1, 1, 1}
}
setCombatArea(combat, area)

function onGetFormulaValues(cid, level, skill, attack, element, factor)
    local levelTotal, formula = level / 5, 0.0496
    local normal, elemental = -(skill * attack * formula + levelTotal), math.ceil((skill * element * formula + levelTotal))
    return normal/2, normal, -math.random(elemental/2, elemental)
end

setCombatCallback(combat, CALLBACK_PARAM_SKILLVALUE, "onGetFormulaValues")
function onCastSpell(cid, var)
    return doCombat(cid, combat, var)
end