"Mirror, Mirror" 'The number of battles where 2 of your parties companions are also on the enemy team' The requirements for this achievement are extremely unreasonable, and it's nearly impossible to get, and will be almost completely impossible in the future when we have hundreds of companions to account for.
I think this achievement should be changed so that it only requires one companion to match the other team(which I've only seen once in hundreds of rounds of gauntlet as of achievements release, so I think it'd still a very difficult achievement to get, but actually feasible compared to both matching.