To get this achievement, you need to play the level "The New House". In the kitchen there is a chance for an "intruder" type anomaly to spawn. He's quite simply a HUGE MAN, with comically large muscles! He'll appear posing, standing with one foot on one of the chairs. There's no way you'll mistake him for any other anomaly!
When he appears, report him as an intruder anomaly, as you normally would. The report will "fail", and he'll have moved much closer to the camera! Now report him again, but this time there's a new anomaly type you can choose specifically for him: "!!HUGE MAN!!"
Send that second report, and he'll disappear, unlocking the achievement for you.