To unlock the Medicine Ball achievement in Little Nightmares 2 you'll need to throw the medicine ball into the mannequin arms that try to grab after escaping the mannequins during chapter 3 (the hospital chapter). The specific part you'll need to escape is the part just after the crawling mannequins. The medicine ball is found on the floor of the room with the electric chair in it (where you pull the switch to release the fuse). Here is my own video guide that shows how to get the Medicine Ball achievement. The guide for this specific achievement is shown at around the 16:43 mark.