Before attempting this achievement, make sure you have garrys mod in a WINDOW and NOT Full screen.
This can be done by going to Options > Video
Now create a new world (gm_flatgrass)
Type the following into console
alias menuloop "+menu;-menu;menuloop"
menuloop
Then MINIMIZE the window. Wait 15 minutes then go into task manager and close out Garrys Mod. (Ctrl + Alt + Del) cause it will be lagging like sh*t