Without any (potentially consequential) Slender Man encounters, this achievement can be easily earned by accessing the Secret Level of the game.
Credit goes to IGN for the following guide (
http://uk.ign.com/wikis/slender-the-arrival/Easter_Eggs_and_...):
Update: Dark Death 90 has stated that
steps 1 and 2 are not required (you still need to collect the three Charlie Matheson Jr. posters, however).
To enter the secret level, select the stage "Prologue" and do the following steps in this order:
1. Acquire the flashlight (randomly located inside the house).
2. Turn on the radio (located next to the downstairs bathroom, adjacent to the garage).
3. Exit the house via the front door and ahead of the garage, you will discover a picket fence.
4. Along this fence you should find a tree with a missing poster for
Charlie Matheson Jr. - collect it. The first time you get this poster, it should say "I should keep an eye out".
5. Restart the level and take the poster once again (the previous steps are not required).
6. Restart the level a third time and take the poster. You will experience static and will be returned to the main menu. The secret level is accessed by starting the game.
During the secret level, you cannot pause; thus, mashing the Start button 50 times will earn you the achievement.
If you are struggling to exit the secret level, there are various video guides on YouTube to help you out.