This achievement was initially unobtainable (as well as Long-term commitment), but it was fixed on March 27, 2017. I'll leave the initial solution I posted and the updates to it under "Spoiler" tags for anyone who might come back to get this later on.
Now it has been partially fixed, meaning that some people will unlock it just fine but it will still not unlock as it should for others. If you are in the second group, please comment your experience in the comments section so that we can identify the issue more accurately.
Update June 10, 2018: If you are still having trouble unlocking this (and long-term commitment), check OdinAsteroid's solution for a work-around. A friend of mine tested it for long-term commitment & it worked.
As for how to unlock it, the achievement itself is quite simple really:
When you load up the game, if you press Start
a list of challenges will appear, the three on the top are daily challenges, the three on the bottom are weekly challenges. For this achievement, you have to complete one daily challenge every day for 7 days in a row. Daily challenges reset every day at 10pm EST (Eastern US) (thank you MathGuy42 for the info). Seven days in a row means at least one chalenge from 7 different sets of challenges in a row. The set of challenges has to reset for the streak to progress.
Daily challenges are usually fairly easy, requirements are along the lines of "Complete 3 blitz matches", "Perform 10 counters", "Play 2 matches with X leader", "Win one match", so you shouldn't have any issues completing at least one of the three challenges you get every day, and pretty quickly also.
Once you complete your seventh daily challenge in a row, the achievement will pop.
Read below if you need info about how the achievement used to be unobtainable: