Ah good point, my bad, well... first thing that comes to mind is the "Assist" command SE said they were going to implement sometime.
Assuming its a given party ofc with a tank & not solo play.

Then again, once you kill one Moo Moo, the targeting circle or arrow will drop from last
current target (and attack will stop or be on hold) until you select a new target i suppose.

I can see "Target Markings" being very useful in a situation as you describe thou,
and i mean no restriction to mark targets as it is now, party / leve only markings sux.

But well thats just my take on how it might work.