I'll be blunt and frank: the objective where you have to kill a varying amount of a specific elite/champion/whatever mob type is so annoying, I tend to just quit to menu and go back in to try another instance, than hunt these down all over the map. It's just needlessly tedious and generally annoying, not due to the enemies themselves, but their widespread placement on the map.

I suggest that they should spawn along an obvious linear path that you can follow and don't have to effectively clear almost the entire map just to reach them in various corners.