The First Do No Harm achievement is one of the secret ones you can earn in Little Nightmares II. It is worth 15 gamerscore on Xbox and a bronze trophy on PlayStation, and unlocking it will help you to achieve 100% completion in the horror adventure title.
This secret achievement can be earned during the final part of chapter three, the hospital section. You will be chased by the ceiling-crawling Doctor, who you have been evading for the second half of the level. Ensure you stay a few steps ahead of him, so you don’t get caught or crushed, and you will eventually reach the incineration room.
Ordinarily, you have to lure the Doctor inside the incinerator chamber, exit through the grate underneath him, and then kill him by pulling the lever to the right of the chamber to ignite it. However, to unlock the First Do No Harm achievement, you don’t want to touch the lever. Lure him inside, escape, and then shut the door to leave him in there. He will try to bust out, but the lock on the door is too heavy for him to budge.
Turn around and head to the elevator, which is how you exit the level. Pull the lever to call the elevator, get inside, and ride it to the floor above. While you wait to leave the elevator, the achievement or trophy will pop with the tagline “It’s crueller to let him live.”