Very easy, just when you start Mission 2 you can get the runes to have the ability to possess a rat at Granny Rags, just go to the basement and get that rune, and do her first side quest.
So at the very start you will see a lot of rats running to the wall of light, try to possess one of them, and go to the tunnel to the right of the wall of light, go to the other side, achievement won.