Support!

Support. Either that, or drastically reduce the number of successes that are required to finish the achievement. Assuming I did my math right (please feel free to double check me; probability is full of black magic), the odds on this are kinda bonkers.

Now I don't know exactly how the gauntlet selects enemy teams, so I'm essentially assuming we're drawing a hand of 5 random companions out of a deck of all 103. If there's more to it, then these numbers won't be accurate.

Edit: and right after doing all this I saw a screenshot where the enemy has duplicates. So yeah these numbers are all off xD leaving it up for now though. Original post below

Edit 2: Okay updated! This is with each enemy independently being randomly rolled from all 103 companions, so duplicates are possible. (spoiler: it's marginally worse now, and definitely don't use two of the same companion)

The odds of having your two companions match and giving you a point towards the achievement are 0.19% 0.09% if you use two of the same companion, and 0.18% if you use two different ones. You need to do this 100 times.

This is already pretty bad, but some of the gauntlet achievements flat out require you to just play it a ton. So I'd be okay with this as long as it shakes out that you'd be likely to complete it in roughly the same amount of time it takes you to do the other long haul achievements. Participate in the Gauntlet (1k; lets assume we reach level 10 on average), Defeat enemies in the Gauntlet (50k; 5 enemies per round), and Win rounds in the Gauntlet (10k) are some of the longer ones and all shake out to about 10,000 rounds at completion, so that's 10k chances to complete Mirror Mirror.

At the 0.19% 0.09% and 0.18% chance of success per round, the odds that we get at least the 100 successes to complete Mirror Mirror during those 10k rounds is ... *drumroll* ..

0.000000000000000000000000000000000000468% (that's 36 zeroes)

With two of the same companion:

0.000000000000000000000000000000000000000000000000000000000000000293% (63 zeroes)

& with two different companions:

0.0000000000000000000000000000000000000194% (37 zeroes)

Okay. Well. Needless to say this'll probably be the last achievement in that category you complete. So how long will it likely take?

For a coin flip's 50% chance you'll have completed it by then, plan to do 52355 107815 (same companions) or 54436 (different companions) rounds.

Want to bump that up to a 90% likelihood of getting your 100 done? 59358 122243 (same) or 61717 (different) rounds

99.9% chance so only the unluckiest of the unlucky won't have managed it yet? 70252 144687 (same) or 73045 (different) rounds

I'm fine with long term achievements, but not when they're entirely rng based. The way these numbers shake out means you'll very likely have already finished everything else, and still be bashing your head against the wall hoping you get lucky. That's not fun, and should absolutely change.

Not to mention some of the companions that can appear in Arena are unobtainable, making it somewhat less likely for those of us not fortunate enough to have them. I get that it's still chance either way, but ouch.

I don't know the statistics of getting it with just needing one companion, nor do I know the math required for that, but at least that'll be within the reasonable realm of possibility. Something like assuming you have two companions and all five enemies per round are different and it only has to match one of them, then if there were still 103 companions (I think there's a few more by now, but I'm not going to find the numbers), it'd theoretically take like... what, twenty runs or so to see all the possible companions? So you could theoretically get one every ten rounds, right now, assuming RNG is exactly even and does not unreasonably bully you. If you only needed to match one. That's... still a lot. But doable. Assuming I know how to math right.

It feels like one of those ideas that's great fun on paper, but then doesn't really pan out. I feel like it should be one of those gimmick achievements that you only have to get a few times due to how unlikely it is. You know, at most.