This achievement requires you to gain 3 gold medals for all species in the game.
It's quite the grind and something you definitely won't have finished by the time you've cleaned up all your collectables.
Once you start grinding this, you'll need to find a stage that gives you 1 millions, fast. The best way to do this is on Secret Stage 06, "Reverse Block 01" and equip the Void Wispon (Equip the version that gives you more points as you collect coins if you have it unlocked)
You can finish Reverse Block 01 in under 25 seconds, collecting over 350+ rings and ending the stage with 1.1m, which is much faster than the other solution listed here.
Here is a video (not owned by me) showcasing the route:
Once you're done grinding medals for an avatar, go into the avatar menu and create a new one of that species, then continue again. As expected, activating a daily mission bonus during this will speed up the process. Good luck!