So what is not stated by other solutions and only obliquely alluded to by the wording of the achievement description is that there are exactly nine allies over the course of the game that require your help in this manner. These are collectibles that make up nine of the game's 23 heroic actions, meaning that despite the game stating "Heroic action performed" every single time you complete one, each heroic action counts exactly once and the achievement cannot be sped up by completing the same heroic action on checkpoint reloads or subsequent playthroughs.
Heroic actions, like mementos, are immediately and permanently collected the moment you get them and are not reset if you die or quit the mission, if the AI you saved dies, etc. Do it once and you never have to bother again.
I will provide a video from PowerPyx to help you get these done since I do not have my own solution for this. A timeline of video links is inside the spoiler.