How to make a move with a chance to 1 hit KO?

class PokeBattle_Move_nnn < PokeBattle_Move [COLOR="Green"]# change to a new function code[/COLOR]
  def pbAdditionalEffect(attacker,opponent)
    return if opponent.damagestate.substitute
    return if opponent.level>attacker.level
    if !attacker.hasMoldBreaker && opponent.hasWorkingAbility(:STURDY)
      @battle.pbDisplay(_INTL("{1} was protected by {2}!",opponent.pbThis,PBAbilities.getName(opponent.ability)))  
      return
    end
    if !opponent.fainted?
      opponent.pbReduceHP(opponent.hp)
      opponent.pbFaint if opponent.fainted?
      @battle.pbDisplay(_INTL("It's a one-hit KO!"))
    end
  end
end