Challenges range from Easy to Expert, and there are 5 challenges each day (1 challenge for each of the 5 different games). You gain coins for each challenge, and bonus coins are awarded for completing an entire day's challenges. Harder challenges award more coins. You'll need to complete a substantial amount of a month's challenges to get the Diamond badge, hence it's worthwhile going for:
And completing every single challenge, as you'll unlock this along the way anyway. If you really want you can pay for a Solitaire Premium subscription, which does award bonus coins and removes ads, but it's not necessary at all.
To speed things up for this it's best to complete the Hard and Expert challenges as they award more coins. Thankfully, Voranc on YouTube religiously uploads the solutions to every difficult challenge on his channel here:https://www.youtube.com/user/Woranc/videos
You can search YouTube for them too, which makes life a little easier:https://www.youtube.com/results?search_query=microsoft+solit
Unfortunately, he seems to delete the entire month's worth of uploads once the month is over... So February's are now deleted. As will March's once we get to April. It's something to keep in mind whilst going for this.
Some of the older solutions can be found on other channels, but they're not quite as comprehensive as Voranc unfortunately:https://www.youtube.com/channel/UCTkORNNzlAW0LCgD1Suh39ghttps://www.youtube.com/user/mscdc4