For the 5th and 10th rounds of Garden Ops a slot machine is used to select which Boss Zombies will appear in that round. There are 4 Boss types: Gargantuar, Disco Zombie, Yeti and Baron von Bats (added in DLC). To unlock this trophy you must first get a Yeti Zombie and most importantly, get the killing blow. The random nature of the slot machine and needed to issue the killing blow can make this a bit challenging to get. Keep at it and it will pop!