The achievement is called "Attention to Detail" which made me believe there must be a difference in their looks... but only if you're paying attention.
I found some comments over at WoWHead that indicate the boss's mirror images are slightly different. I will copy in some of these comments.
Option 1: Look closely!
You can look at them and pay close attention:
Black hair that goes over her shoulder,
BAD VARIATIONS INCLUDE
Option 2: Macro/chatbox solution
You can also make a nice macro (or type a script into chat) that will show you the model of the original boss alongside a model of your current target, which should help with quickly identifying the right one:
First, this one:
/run frm = CreateFrame("PlayerModel");frm:SetPoint("BOTTOMRIGHT",nil,"BOTTOMRIGHT",0,100);frm:SetHeight(600);frm:SetWidth(300);frm:RegisterEvent("PLAYER_TARGET_CHANGED");frm:SetScript("OnEvent",function(self,event,...) frm:SetUnit("target") end);
It will show a huge model of your current target at the right of your
Then, target the boss and use that one:
/run frm2 = CreateFrame("PlayerModel");frm2:SetPoint("BOTTOMRIGHT",frm,"BOTTOMLEFT",0,0);frm2:SetHeight(600);frm2:SetWidth(300);frm2:SetUnit("target")
It will display the original boss next to your current target.
After that, the achievement should be easy
And to deactivate the model box: