Item description:

This save unlocks the Infused with Greatness achievement - awarded for collecting all of the infusion upgrades in a single playthrough - It will unlock some other combat related achievements as well as you have to complete the scene to get to the infusion.

I can't do anything about this as the bioshock autosaves in checkpoints and the last one is just before the event.

Simply finish the scene and as Elizabeth and you run up the stairs to get to the elevator, instead of going left and into the elevator, turn right and get elizabeth to unlock the door. Inside is the last infusion.

Tested on my dummy account and it works fine, any problems let me know.