Broken Path (Eden's Convent)
After liberating Eden's Convent in Faith's territory, speak to the starred NPC inside the chapel.
You now have to defend the convent against waves of cultists- a timer at the top of your screen will give you a countdown to the first incursion. You can deploy explosive first if you like, or need the challenge kills, but the mounted gun atop the trellis works fine on its own.
The next wave comes from the west, but luckily there's a west-facing gun just a stone's throw away. Cross over the roof to get there.
You'll again be gunning down cultist hordes, but keep a particular eye out for snipers, who like to perch on the rocky outcrop to your left.
The last wave will come from the south, in theory. Cross back over the roof and man the gun down off the patio.
Note that, between the incoming helos and stragglers from previous waves, not all your enemies will actually be in the south. Luckily this gun has a 360 turn.
Take down all hostiles, including four helicopters, to complete the mission.