In order to unlock this achievement, you will need to join a game of Gardens and Graveyards as a Zombie (classic or normal, both are fine, but I recommend classic so it's harder for the players to vanquish you) and find a pile of dirt and bones on the ground. Simply go near it, press
to open the selection menu, select which zombie you want to summon, repeat these steps 4 more times, and boom, achievement.