Severed Ties

Bonus points if you use Sylvanas' Music Box during this.

Earning the Achievement:

Although it's not clear from the wording on the achievement, you must kill two ghouls in a single Highborne phase to earn this achievement. Simply killing one ghoul in each Highborne phase - two total - in insufficient.

You can kill two ghouls in any single Highborne phase, though, and earn the achievement. And it only has to be done in one of the Highborne phases, not each phase. If you kill two ghouls in one phase and only one ghoul in another phase, you'll still earn the achievement when the boss dies.

Best practice is to mark an area on the ground, and plan to kill the two nearest ghouls that spawn to that point, unless you're very fast with putting up raid markers (a raid marker on the group may help, especially since some group members may get turned around and disoriented when she pulls the group in). The boss doesn't hit hard, so some groups may even use a hybrid to tank (e.g., a dps kitty in bear form) to have extra dps to burn the two ghouls for the achievement. A reasonably well geared group can do it with the standard 1 tank, 1 healer, and 3 dps. Save personal and group dps cooldowns for the ghoul phase.

Hint: rather than try and mark a specific pair of ghouls (which costs dps time), put a ground marker in advance with instructions that players kill the nearest two ghouls to it.