Oh Fiesta~!

[Yosei]

That's true...but then again if this was implemented couldn't someone just hit every monster in a map once, run away but stay in the same area, then someone else unknowingly kills them all and the first guy gets all the exp? That'd be the end of the world.

And Fiesta's situation is better? :P All they have to do is simply make it so a mob will start to gain HP back if it goes too long without getting hit, thus it is considered no longer in combat and whoever actually works to take it down gets the exp.