Far Cry 4
57 Achievements
1,250
44-55h
Defender
Repel 3 Outpost Retaliation Parties (Campaign only).
15
32.39%
How to unlock the Defender achievement in Far Cry 4 - Definitive Guide
It's unuseful to wait near the outpost, because it's random and you can wait a lot for nothing.
Since outpost are also quick travel point, once or another going away from the outpost you'll be advised to run back and kill all the bad guys.
Actually you don't have to kill them all, just a few (I never failed on this): after a certain number of foes in the dust the survivors will retire and after 3 times you'll unlock the achievo.
UPDATE (thanks to lxl Linc lxl)
This is also missable if you liberate the fortresses before getting 3
UPDATE 2 (thanks to xDeMiZEx86 )
If you conquered all the outpost and all the fortress, you can still reset them all even if you're in the post-game
Since it is the only outpost you have, the game will always pick that one.
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.
In order to earn this trophy, you must protect an outpost that you've liberated from enemies that attack it. For more information on how to liberate an outpost, refer to All Clear for more details. You must protect your outposts 3 times.
Enemies attacking your Outposts will be at random, however, there is a way to increase your chances of getting attacked. Liberate as many outposts as you can. Also fast travel to an Outpost from your map on the menu. It seems that whichever outpost you've fast travelled from, is most likely the outpost that will be under attack so keep fast travelling to different outposts until you get attacked. Also bear in mind that you will need to leave the Outpost a certain amount of yards until you're attacked. You will not be attacked if you're near an Outpost, so walk/drive away a fair bit until you get attacked. You will know when an outpost is being attacked, as you'll get a notification saying: "The nearest Outpost is under attack". When you see this, immediately turn around and head back to the outpost. You will also see a yellow waypoint so you'll know which outpost is being attacked. You will not be able to fast travel to the outpost so make your way back manually. When you reach the Outpost, enemies will ambush the area. Kill every enemy until you complete the retaliation attack.
Also bear in mind that liberating a fortress will cause enemies to stop attacking your liberated outposts in the area for the conquered fortress, so do not conquer all 4 fortresses until you meet the requirements for this trophy. If you've conquered all 4 fortresses, do not worry, as you can reset all you that outposts/fortresses from the menu under Gameplay (Campaign), there is an option to "Reset Outposts". This can only be unlocked if you've liberated all 24 outposts however. Not only that but you'll have to liberate a fair amount of outposts all over again to better your chances of getting attacked, therefore it's highly recommended you save at least one Fortress until you earn this trophy. After you get the trophy, you can just ignore any of these quests that appear so just drive away and it will eventually disappear. It will not effect the outpost, or karma.
Repel 3 such attacks and the trophy is yours.
This is exactly what happened to me. Fortunately, in the settings there is an option to reset all outposts and fortresses.
There is no need to worry about the achievement; by that time I had liberated only 6-7 outposts in total. Quite often you will be invited to defend. Current statistics on repulsed attacks of punitive squads can be viewed in the menu during the game, in the “progress” section (at the very bottom.)