This achievement is glitchy and might not unlock as it should even though you’ve completed the placement matches and have gotten your ranking.
It seems to not unlock as you complete it, but rather the next time you connect to the versus online services. If it glitches for you and you want to force it to pop, try to:
- quit out of the game completely
- start the game up again
- choose the “Versus” menu option
It should pop right as the game reconnects to the online services. YMMV though.
This tip was mentioned by many in the comments to the other solution, for example by Uh0h its Renaud