The Daily challenge can be found in the top left of the home screen. This is a cumulative achievement, meaning you do not need to win 25 sequential daily challenges, but just a cumulative count of 25 wins. You can only play a daily challenge once per day. If you lose the challenge, you have to wait till the next day to try again. The achievement will take you a minimum of 25 days, but this is not the longest achievement grind in the game....