Once you've captured an outpost, enemy troops will periodically attempt to reclaim it until you've captured the fortress in the area.
This normally seems to trigger when you are near an outpost that you've liberated (200-400m away or so) but not actually inside it. You'll be told it's under attack, then you need to head back and take out enough of the enemy troops to repel the attack. You'll know when this happens because the enemy will generally start yelling 'retreat'.
Repeat this x3 and you're done.
Note, this is potentially missable, if you capture all 4 fortresses before you've won this trophy then no more retaliation parties and you've probably wrecked your chances.