You have to survive two scripted events for this achievement to pop.
First: The queue to enter the event.
You will be approached by some NPCs to talk, and you can just ignore/deflect/avoid saying the wrong thing but there is a trap there. By the end of the line, where you see two women discussing and two men watching you, you
MUST NOT interact with the TWO MEN, or they will give away your disguise.
Second: The Commander competition
You will have to interact two times with the dance/ceremony area where you will face your copycat, you, again, MUST NOT use your mythic related options, you gotta stay hidden, blend in.
And when Razmir tries to taunt you, ignore it and play it cool, Rovus got your back.
The achievement pops when you get inside Razmir's palace.
Credit to ALEC for their
guide.