When you're in close proximity to Slenderman, your video will shake and flicker. Pressing Menu/Start at this time will not pause the game. To get the Achievement, just mash the button like crazy while also moving away from Slenderman so that he doesn't kill you.
Hitting the button 10 times during one encounter shouldn't be a problem. If you're quick, you can even hit the button fifty times to get 'Why am I even playing this game?' during the same encounter.