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 'Cats versus Dinos'
Cats versus Dinos is an elimation style deathmatch where death means you are out for that round. Teams compete to win the best out of a series of 5 rounds.
For this achievement all you need to do is complete a full game when you are on the cat team.
You do not have to win and you can die.