Downfall

To get him really mad, when you meet him for the first time, use the option "You are full of crap" then agree to go down to the cellar.

After you get back, be rude to him then tell him you are sure he is MAD then proceed to tell him he is full of crap again.

Achievement will unlock 😃

If you dont usually go into the options menu, you wont notice the option to turn the autosaves off.

So before beginning the game, go to the options and turn the Autosaves off.

Turn off the hints too while you are at it to get the "Hardcore" achievement too.

This is the trickiest part of the letter to find, I did miss it on my first playthrough.

This is a time-scripted event and you can totally ignore it if you are not careful.

After reading the first part of the letter and exploring the second floor, you will hear faint whispering in one of the corridors.

There is usually nothing to see here so you would first think to keep walking but what you have to do is stop and wait to listen.

After a very fun event, you will find the second part letter on the ground.

Go to the extras in the options menu and try the lucky dip option.

Do this several times until the achievement pops.

While in the maze as Agnes you will find a Doll which you will use as a brush later, but dont do that just yet.

Right after picking up the doll, go outside the Maze into the woods, you will find your friendly neighborhood Slenderman and this achievement will be yours.