This is earned in Act 5, Part 2 when you’re tasked will saving Will and a timer pops up.
It is missable.
Once you reach the door the leads to where Will is, you have to let the cutscene that begins play out. If you skip the cutscene, it won’t unlock. The achievement will unlock once the cutscene is over. You can just redo it if you did skip the cutscene.