So, a few thoughts/suggestions:
On another thread discussing this achievement I pointed out how the staff are unlikely to change it to one due to the immense drop in difficulty. Coming across one matching companion is actually far easier than you would think. I would have completed this achievement in a month, if that, if we changed it to one. (Editing to add, I wouldn't be upset if the staff did change it to one, I just think it's very unlikely that they will. I am still partially in support of this suggestion.)
An alternative I saw someone suggesting previously was at least lowering the required number of instances to get the achievement. Maybe cutting it in half would make the community happier? Then each of the five tiers could be just gaining ten additional instances of duplicate companions.
A suggestion I made was introducing new Gauntlet venues with groupings of specific companions in each one following some sort of theme. That way, the pool of possible companions one would encounter would be smaller, making it easier to choose two companions that might appear on the enemy team. We could have the main Gauntlet venue with all of them, like what we have currently (minus certain newer additions), and then the smaller sub venues with smaller companion pools.
Statistically speaking, this achievement is a bit of a nightmare, especially with new companions being added somewhat regularly (thankfully, not every new companion has been added to the Gauntlet pool, but it's only a matter of time). In the future, without any changes made to the requirements, it will be far more difficult for users to achieve, especially newer players or those who don't have the time or resources to spend on Gauntlet battles.
Speaking as someone who has been grinding this achievement for the last several months, even if I finished it and saw the staff lowering the requirements later, I wouldn't be mad. I would have achieved it either way, and others who are getting close to the end might become eligible to instantly finish it depending on how far they progressed.
That all being said, even if it seems impossible or daunting I have gotten 70/100 total instances, so believe me when I say that there's hope! But I recommend working on this one sooner rather than later if you are concerned with completing all your achievements, because this one will definitely become harder as time goes on.
Updating to say that after one additional week, I've gained 5 more instances for a total of 75/100 total instances of Mirror, Mirror! Hopefully this encourages others to keep trying. Always remember that progression is possible. <3