At the very beginning of Chapter 3 you'll have to go through a broken bridge.
Use Spot to reach the opposite side, let him move the broken bridge and then ask him to drink water at the fountain in order to be heavier. Now move Spot on the bridge.
A mini game begins and you have to move all the people in just 8 moves (Spot is excluded):
1- move Renie
2- move Noah
3- move Sam
4- move Spot back
5- move Janus
6- move Renie back
7- move Keira
8- move Renie once again
Achievement will unlock immediately after.