Mystery portals are timed community events that appear at scheduled times.
Go to http://www.pvzgw2.com/events-calendar
to find out when they appear.
Mystery portals feature only one gamemode at a time and this achievement can only be earned when the gamemode for that specific event is 'Super Mix'.
Super Mix is a custom playlist where Crazy-modifiers are active such as teams being limited to only a few characters or modifiers such as superspeed, low health etc enabled.
For this achievement all you need to do is complete a full game.
You do not have to win.