I was doing some mythics earlier and it seems there is an addon, Weak Auras and something else, which would place a Moon over a particular target out of a pack of enemies (in this case the one with highest HP because of the Bolster affix).
I was trying to imagine how it might work.
I can only guess that when you go in combat, there will be events about enemies aggroing you or your party members ? And then the addon must be using some API to place a target marker over a specific target ? And even if there are multiple targets with the same name, eg "Crazed Disciple", I guess it still somehow works and can tell who is who and mark the right one ?
I mean DBM puts markets on players, so I presume an addon can place a marker over an enemy ? The clever bit is that the marker is placed on the pull before you even have the chance to hit anything or to be hit, so it works not off of your attacks, but on some kind of aggro message/event.