The Ace Tanker mastery badge looks like a gold medal with a large M on it. It's for scoring in the top 99% of tankers using that tank, since the last server reset of all the players.
The servers keep a rolling 7-day total of the best players for the last seven days. Therefore playing on a Monday/Tuesday is possibly better because it will now start excluding all the weekend play from the previous week in the total and you have the best chance of hitting the Ace Mastery if not as many people did well since the last weekend in the tank you're using.
But it really comes down to doing well in a tank and beating everyone else's scores. It will be very variable because the tank you're using might've had some very high scores either the previous week or the last weekend--you'll just have to try. Choosing a little used tank will also greatly improve your chances of getting the Ace Tanker!
I did this with a number of tanks--I really like the US Tier 1 T1, Tier 5 M4, and many others. You just basically should use a tank you really like and are good with, and aim to outscore everyone else who used that tank since the last server update. You can get this over time, or if you want to specifically try for it, then stack equipment and maybe even premium ammo/consumables just to give yourself an edge in the fight.
If you manage to get 5+ kills, you're almost guaranteed to get the Ace Tanker badge.