The Mirror Mirror achievement requires having two companions on your party also appear on the enemy team, which given the number of companions currently in the game and the fact you can only have two companions on your team at any one time, is an exceedingly rare occurrence. I haven't been able to get a single point toward the achievement despite usually running the Gauntlet at least once per day.

My main concern is that as new companions are added to the game over time, the odds of meeting the conditions for this achievement will continue to drop, making something already quite difficult into something nigh on impossible. I think the achievement would be better if it was adjusted to having just one of your companions on the enemy team, as it would still be rare but at least is more feasible to encounter with a 3-wolf 2-companion team composition.

Even in this case it could still become quite difficult to get the achievement in future years when the amount of companions increases, but at least for the foreseeable future I think this would be better than the current state of the achievement. Please feel free to share if you have a different opinion or different experience with the achievement :)