Unless the achievement "Mischief Managed" is meant to be exceedingly rare, it is probably set at too high of a number. Going through the 5*3 levels, Ascension to 20, and 5 years of Endless got me to 5/10, and that was with the plan of having as many monsters as possible in Endless specifically to farm that achievement.

I now have every possible monster in Endless and seem likely to get this last achievement by continuing this Endless run as long as possible, hitting as many events as possible. 2-5 hours left to get it?

Setting it to 5 stolen artifacts would turn it into something possible to get during an otherwise 100% completion. Setting it to 3 or 4 would turn it into something possible to get during normal play. At 10, it is a longer grind than the second hidden Endless achievement.