you can't fight over your own level. you can use /whistle to lower the mob level. The main use of this is for healing, lets say you're level 55 with a mage with 6000 mana left but only 300 hp of 1600, you could /whistle 1 heal up, then reset the whistle to your own level or explore.
the whistle level will follow your own character, so if you level up it should also level up the whistle. So just spam the hotkey t