What the achievement asks
Defeat So'leah in Tazavesh after being caught using forged credentials, on Mythic difficulty. Ten points, account-wide. It gives the Tazavesh Gearglider mount, which is the reason anyone does it.
The credentials are the whole achievement. Killing So'leah is the easy part.
The run happens on two difficulties
This is what makes it awkward: the preparation is done on Heroic, the achievement is earned on Mythic, and you have to leave and reset between the two.
On Heroic
| 1 | Enter and kill the first boss, Zo'phex the Sentinel. |
| 2 | Head right, down the stairs. In a corner, a Questionable Trader sells Fraudulent Credentials. |
| 3 | Carry on to Au'Myza's Oasis. Just past the stairs on the left is Au'manal. |
| 4 | Turn on tracking for low-level quests, and take the quest he offers. Skip this and the guards stay hostile later, with nothing explaining why. |
| 5 | Equip the Fraudulent Credentials and speak to him. They become Passably-Forged Credentials. |
Then on Mythic
| 6 | Leave, reset your instances, and go back in on Mythic. |
| 7 | Before moving, use the Passably-Forged Credentials to take a broker's shape, and speak to Fe'tajid. He stands at the very beginning of the instance, not beside Au'manal, and he only appears on Mythic. |
| 8 | Exhaust his dialogue. That lets you talk to the guard and walk in rather than fight. |
| 9 | Clear the dungeon and kill So'leah. |
If you do not speak to Fe'tajid, you finish the run and get nothing, and you wait for the weekly reset to try again. It is the single step people miss.
Two things that go wrong
The Grand Menagerie can break if you kill it too fast. Take the first two of the three slowly, wait for the pandaren to drop in, and only then push. Killed at speed, the encounter can fail to complete.
So'leah takes almost no damage in her second phase when you are alone. Ninety-nine percent reduced, so a solo kill is measured in minutes rather than seconds and there is nothing wrong with your character. It is the single longest part of the run, and it is simply a matter of waiting it out.
What you also get
The same kill awards Tazavesh, the Veiled Market, the ordinary Mythic achievement, so the two are done in one visit. That one is per character; this one is account-wide.