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 ten allies over the course of the game that require your help in this manner. These are collectibles that make up ten 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 ten struggling allies are found in the following missions:
D DayStruggle #1 -
0:05Struggle #2 -
0:22Operation CobraStruggle #3 -
1:27Collateral DamageStruggle #4 -
5:30Struggle #5 -
6:06Death FactoryStruggle #6 -
7:47Hill 493Struggle #7 -
8:25Battle of the BulgeStruggle #8 -
9:19AmbushStruggle #9 -
10:51The RhineStruggle #10 -
12:03A complete timeline of heroic actions is inside the spoiler.
*** Spoiler - click to reveal ***D Day - 2 heroic actions
Struggle #1 - 0:05
Struggle #2 - 0:22
Operation Cobra - 3 heroic actions
Surrender #1 - 0:41
Struggle #3 - 1:27
Surrender #2 - 1:46
Stronghold - 1 heroic action
Drag #1 - 2:09
SOE - 1 heroic action
Drag #2 - 2:53
Liberation - 2 heroic actions
Drag #3 - 3:30
Surrender #3 - 3:59
Collateral damage - 3 heroic actions
Drag #4 - 4:25
Struggle #4 - 5:30
Struggle #5 - 6:06
Death factory - 3 heroic actions
Drag #5 - 6:34
Drag #6 - 7:09
Struggle #6 - 7:47
Hill 493 - 2 heroic actions
Drag #7 - 8:02
Struggle #7 - 8:25
Battle of the bulge - 2 heroic actions
Drag #8 - 9:00
Struggle #8 - 9:19
Ambush - 2 heroic actions
Drag #9 - 9:59 (I've Got You unlocks here)
Struggle #9 - 10:51
The Rhine - 2 heroic actions
Surrender #4 - 11:24 (Quarter Given unlocks here)
Struggle #10 - 12:03 (Rescuer unlocks here)