I posting this as a solution, so it does not get buried in comments. I did this during Season 7.
First, I had to obtain rank 20, which I did playing Co-op vs. AI. It took about two hours on a weeknight.
Next, I played the 2vs2 Gnasher under the Competitive and the TDM under Core. I did five games under both those playlists over a weekend. After the fifth game of TDM, the achievement popped. It did not give any tracking prior to the achievement unlocking. The Gnasher matches would take about 2-3 minutes to match me up, but it would match me up. After the achievement popped, it showed 12% Completion towards Seriously 4.0.
Finally, I don't know what to expect with the January update, so I will update accordingly. UPDATE: As of 06 February 2020, there has been NO January or Final Update. Season 7 is still on going.