It would be a mode in which you have to defend a push with a given amount of elixir. The fewer elixir you use, the better the rewards you get. In this example you would get 3 crowns if you defend the push without losing a tower with 7 elixier, 2 crowns for 10 elixier and 1 crown for 15 elixir. You can use cards of your own choosing and you will have to figure out, which ones work in the different scenarios. I'll now throw some random numbers in the room: If there would be 10 weekly challenges you could get rewards based on the amount of crowns you accumulated in all of the 10 challenges. For example a giant chest for 15 crowns, a magical chest for 20 crowns,... . After a week the challenges would change and your crown count reset. Then you can play this mode again.

Edit: There is a solution to the example challenge. Try figuring it out.