Why lose experience for fleeing anyways? Not everyone has the skill retreat, GM's Warrior's and Rangers only. So... come new code unless there is another way to escape a battle or retreat is given to all classes, we'll always flee on tough mobs and therefore always lose experience...
But here is an idea, every time you flee, you lose experience... like now. However, the total experience received from the kill has those flee's tacked onto it. So let's say Gojira becomes more ultra than he is now because we will undoubtedly need to group to kill him. And say everytime the group flee's his experience becomes more and more. This group gives up and another group comes along and kills him and they get all that extra experience because the mob was just that good. This solution gives the individual a chance to get that exp back if they finish the mob off, or just take their losses and give someone else the benefit of their loss.
...just a thought.