I was playing this regularly on the daily for a few months. I got all the other achievements but this one and the character level 20. I started doing time skips by going offline and moving my clock forward a day on March 20th and it took until December 30th to get a level 15. I find the gold card reward at the end of the 16 day calendar is most likely the card you will get to 15/20 first, as there are only 3 possible card from that draw.
Three months later I got the level 20, and had only gotten that same character to level 16. What gets you to level 20 fastest (and probably even level 15) is the TPS reports.
For the TPS reports, try to get as many characters as you can until you unlock the tier that gives you 25 of the currency. At 25 the grind isn't too bad from there to get the level 20 card if you focus on it.