What the achievement asks
Defeat So'leah in Tazavesh, the Veiled Market on Mythic difficulty. Ten points, both factions.
This one is not tied to your account, like the eight dungeon achievements beside it. It has to be earned by the character that runs it.
A keystone run counts, despite what the wording says
The description names Mythic difficulty and says nothing about keystones. When the achievement launched that was literal: people cleared Tazavesh in Mythic Plus and received nothing, and Blizzard acknowledged it without offering a fix.
It was corrected in patch 9.2.5. A Mythic Plus run now awards it like any other. If you read an older guide saying otherwise, that guide predates the fix.
Tazavesh is a megadungeon
Eight bosses rather than the usual four, and it is the only dungeon of the expansion built that way. So'leah is the last of them.
| Order | Boss |
|---|---|
| 1 | Zo'phex the Sentinel |
| 2 | The Grand Menagerie |
| 3 | Mailroom Mayhem |
| 4 | Myza's Oasis |
| 5 | So'azmi |
| 6 | Hylbrande |
| 7 | Timecap'n Hooktail |
| 8 | So'leah |
In Mythic Plus the dungeon is split in two halves, and So'leah belongs to the second one. A key on the first half, however well it goes, will never award this achievement.
The mount is a separate achievement
Killing So'leah normally gives you this one and nothing else. The Tazavesh Gearglider comes from Fake It 'Til You Make It, which asks you to reach her a particular way, with forged credentials.
Since both end on the same boss, it is worth reading that one before your run rather than after: the preparation happens on a different difficulty, before you ever enter on Mythic.