Seriously POP! has four separate objectives that must be completed. I would recommend going for each objective in the order listed in this guide for efficiency.
Because each objective's individual solution has so much detail which makes the overall achievement solution very long, I have put each objective solution in a spoiler tag.
Also, the achievement is glitched in a positive way, as confirmed by myself and several commenters, as the in-game tracker seems to award partial credit for certain objectives randomly without needing to actually perform the required task.
Unfortunately, the in-game tracker does not do a good job of breaking down the individual requirements. Instead, it displays you needing 102,001 "points" to unlock. Theoretically, 100,000 of those is for Pins defeated, 1000 is for Versus wins, 1000 is for Horde waves cleared, and 1 is for reaching Gold League. However, you may not need a full 1000 Versus wins or Horde waves, since these are the two objectives that seem to be positively glitched (see the comments for reported incidents).
Therefore, I would recommend you to check your in-game tracker for progress as best you can to avoid wasting time grinding for Versus or Horde waves, though it will be very difficult to parse out wins/cleared waves registering/not registering with the current tracker.1. REACHED HIGHEST LEAGUE
2. WON 1000 VERSUS BATTLES
3. CLEARED 1000 HORDE WAVES
4. DEFEAT 100,000 PINS
If you have any questions or suggestions, please let me know in the comments and I will update the solution appropriately. If something in the game is updated and renders portions of this guide obsolete, then again, please comment or message me so I can update the solution.