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 four (very small) groups of enemies over the course of the game that will surrender. These are collectibles that make up four 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.
The four surrendering groups of enemies are found in the following missions:Operation Cobra
Surrender #1 - 0:41
Surrender #2 - 1:46Liberation
Surrender #3 - 3:59The Rhine
Surrender #4 - 11:24
A complete timeline of heroic actions is inside the spoiler.