You might be able to use "mouseover" instead of "target" and the event "UPDATE_MOUSEOVER_UNIT" but it is still reliant on the mouse moving over the mob and will trigger whenever you mouse over anything with a 3d model.
https://warcraft.wiki.gg/wiki/UPDATE_MOUSEOVER_UNIT
nUI uses mouseover if I remember rightly. At least I remember the odd rare message pop up as I moved around the world with my mouse.