Complete every single daily challenge in a month. This does not have to be done on the correct days - you can do every challenge on the final day of the month if you want, since you can select previous days' challenges from the "View all [Month] Challenges" icon to the top right of today's 5 challenges. You can also change your system date and do a previous month's challenges (your achievement will be timestamped at the changed date). Along the way to this achievement, you'll also pick up a Bronze, Silver, Gold, and Diamond medal, helping towards the achievements for those.
Challenges range from Easy to Expert, and there are 5 challenges each day (1 challenge for each of the 5 different games). So you're looking at around 150 challenges in total, give or take a day or two's worth.
The difficulty in this achievement is essentially completing the Hard and Expert challenges. Thankfully, user Voranc
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. Therefore, you'll need to at the very least complete all the difficult challenges for a month before he deletes the solutions. You'll then be free to go back and complete the easier challenges at your own pace (note: according to Maliicious in the comments you can only go back 2 months
). Irritating, but all the hard solutions, if only for a limited time, is better than nothing.
As Falcon says in the comments, 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