Attention to Detail

I made a bunch of macros to help with the achievement. Just Copy/Paste the following in your chatbox

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 screen.

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:
/run frm:UnregisterEvent("PLAYER_TARGET_CHANGED");frm:Hide();frm2:Hide()

EDIT: Oh boy. It's not 9 illusions, I just did the dungeon on heroic and the difference is the illusions spawn in 3x5 making it a total of fifteen clones at once, still with only one real one. Good luck!

As mentioned above and in the Dungeon Journal, Janice Barov will disappear at 66% and 33% and summon nine illusions set in rows of 3x3 before then hiding behind the one that looks just like her. Each illusion will cast a different [spell=113775] that is AoE (feintable) Arcane damage and there is absolutely no difference in spell animation between them. The clone she hides behind is completely unique to the others, the other 8 have different variations and even are sometimes complete copies of each other but never the real one. If you miss an illusion, you will hear a taunting /kiss and have one less clone using [spell=113775] on you but the achievement will fail.

The only way to truly do this achievement is to click each portrait until you find one with a black headband, long black hair that goes over her shoulder, a black chestpiece and black shoulders as these are the only things you need/can get from clicking on portraits. The hardest part to see is the chestpiece and is seen just below her chin. Remember, you only have to hit the right illusion twice, once in each phase. The illusion phase lasts until you hit the right illusion so, if your group is geared/prepared enough, you can take more time to hit the correct one while surviving the massive AoE magic damage by rotating cooldowns.

Just to Clarify:
CORRECT ILLUSION
Black headband,
Black hair that goes over her shoulder,
Black chestpiece
Black shoulders

BAD VARIATIONS INCLUDE
White shoulders
White hair
Short hair
Sapphire-White headband
Ruby-White headband
White Chestpiece

Easy Achievement.

1) Clear all adds in entire room, skeleton shelves as well.

2) Pull boss and fight her at the green candle.

3) DPS her down, she will use her illusion.

4) Hang out by the candle, her illusion will fail and she'll come a runnin' on back.

5) DPS her down, achievement complete.