I am not that good at writing these guides but since there is nothing else better here at the moment, I will do my best to help as many as i can before someone else comes along to do a better job.
Behind the spawn where the goat is in the cage, there should be a large orange building called the golden tower. The easiest way to get there is to go to the larger hotel building to the left of the orange tower and walk into the elevator on the right side of the room. This will take you to the top of the tower. Now, if you look off to the side of the building, you will see the golden tower with a bunch of green spinning goo containers on the top of it. You want to jump to this area of the golden tower. I would recommend double jump and angel goat mutators, but there are multiple other ways, i.e. using a hang glider. Once there, you will need to jump on the electric bed that the scientist is standing next to in the middle of the roof of the golden tower. This will unlock the prototype goat mutator.
What I did to get the achievement was go to the mall and infect all non-zombies in there by using the left bumper to baa on them. Once you have at least five, i quaralled them all into one area, after which I jumped and pressed y (with prototype mutator activated) and I got the achievement.
Hope this helps for the time being.
For some reason, the inventory goat was not an available mutator in GoatZ. While i had it here and there in the MMOre Goats DLC, it was not an available mutator for me to select. If you are able to use the Inventory Goat mutator, it may prove to be easier than quarreling them into one area.