[ Update may 2023]
Although the event is not officially discontinued and the achievement therefor not officially unobtainable, it is worth noting that the last 'Cats vs Dino's'-event took place in december 2019. There is no indication if and when the event will return at some point in the future. [Guide]
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 Dino team. You do not have to win and you can die.