This achievement is earned at the final junction at the end of the fourth act where you play Paul one last time. During this junction you need to decide whether you want to take the final, possibly damaged, dose of medicine and try to see your plan to completion, or if you want to skip the medicine and let your enemies fight amongst themselves. You get this achievement for choosing to take the medicine.
Note that you can make this choice, then immediately replay this section and make the other to unlock both achievements if you don't want to worry about replaying it toward the end of the game. Make sure you do not replay a junction choice until you have completed all of the collectible achievements since doing so will change/move many of the collectibles, essentially requiring you to refind them.