Gems of War

To earn this, you will be completing Explores in the kindom(s) of your choice. For fewest battles you'll want to pick the kingdoms with the extra mythstone bonus (+20%) but it isn't required for this achievement, so feel free to explore where you need traitstones or where you get some other benefit.

Each boss chest now has a chance to become a Mimic chest instead. If this happens, you'll fight another battle against 4 Mimics. The Mimics spawn at Level 150 (E12) regardless of the difficulty you were on during the explore! If you win this battle, instead of getting your token(s) you'll get twice as many tokens. You'll also be 1/3 of the way to this achievement.

That's it. Explore, find Mimic chests at the end, and then beat them three times cumulatively and you'll have this achievement.

The odds aren't fully known yet, but I did 13 explores to find my 3 mimics (3 mimic, 10 not mimic/regular). Since then I've been less lucky. After nearly 100 boss chests my sample size is getting to be large enough to make a proper estimate. My current estimate is that the mimics will appear instead of the boss chest about 10% of the time.

This means, that on average, you should expect to need to complete 30 explores with a boss chest to earn this achievement--and many, many more for the next in the series. As with all things average, some people will get this much sooner (like I did on only 13) and some will not get this until 100 boss chests, or more.