Revenge of the Titans - Drunken Family Argument achievement

So I've played the Xmas mission three times now and I've only gotten a total of 4 Ultrayetis appearing over the course of the mission. As far as I know, I need to take down 5 Ultrayetis to get the achievement. Am I doing something wrong or is it a random amount of yetis each time? Thanks!

(Is there a better subreddit for this question..? xD )