Doesnt matter what character you pick. Need to have a zombie consumable/card so buy the first pack in the sticker shop just in case. Then in game look around for a little dirt mound that will be highlighted and have a "b" interaction. should bring up all the zombie cards u got unless its a bot turrets mound which look similar. After u pick the card the achievo will pop. Summon 4 more for Zombie Friends achievo.