I currently have a challenge to win a stockpile match.
First, BTB challenges shouldn’t be in the game because getting into a BTB match is an exercise in frustration. It usually take several tries just to get into a BTB match because of constant join errors (and other misc errors).,
Second, even if the BTB matchmaking system wasn’t horrible, it would still be random to get into a stockpile match. It takes too many attempts to get the right game.
Also, winning a match should never be a requirement. The assumption should be that most players are playing solo and needing to win a match for a challenge (especially BTB, especially a non-selectable game mode) is just telling your player that you have no respect for their time.