To unlock the ...And Stay Dead achievement in Little Nightmares 2, you’ll need to kill and proceed to beat a reanimated hand with a sledgehammer on the Hospital level/chapter three.
After finding the key inside one of the stuffed animals and unlocking a door upstairs, you will be boosted by Six into an area with a dismembered body. The body’s hand will suddenly shoot off and run and hide before trying to attack you in the next area. Run through the next couple of rooms, avoiding the hand, before climbing up and through a vent. Once out of the vent, you’ll fall onto a body in a body bag. Walk up to the open door at the end of the corridor and into a new room. You’ll hear the hand moving above you. Keep working your way through the room, and the hand will eventually drop down. Avoiding the hand and heading left, jump up on a table and through the shelves. In this next room, you’ll see a hammer on the floor. This is the somewhat tricky part. You’ll need to time your attack just as the hand is curling up for a jump. Hit the hand three times to kill it. Once it’s dead, beat it three more times (just to make sure) and the achievement will pop.