Walk the Path
This mission occurs immediately after you kill Faith in Paradise Lost. Enter her bunker, and follow your marker through the mists.
As in the boss fight, a single bullet dispels illusions.
Catch up with the Sheriff, who's in a bad way.
One thing to keep in mind: despite the high stakes, no part of this level is timed- so don't loose your head when the bullets start flying. This will happen very soon, as you follow your objective to a room with five valves (each one with its own plot marker).
The bow is again useful as a relatively quiet roomsweeper, but every time you turn another valve, more enemies will pour in. You can either tediously kill each wave, or go for a five point sprint. If the latter, make good use of slides, and be sure to work from the near end of the room to the far right corner- that's where your exit will be.
When all five valves are turned, your objective switches to the door in the southeast corner. Like all the sealed doors, this one takes a few moments to open, so consider deploying a smoke bomb if the room is still full of hostiles.
Go through another sealed door, and take out the two hostiles right in front of you.
Advance slightly and look right- there a gap in the balcony here that is perfect for taking out the other hostiles here, and destroying all three pumps (shoot anything that's red, or just pitch a grenade).
The rest of this level is shooting hostiles and moving past timed jets of fire. Again, despite the frantic nature of the situation, there's no actual time limit here.
Be especially cautious until you've taken out the two heavies in the first mob you pass through. After that it's smooth sailing, with only sparse groups of normal fighters.
Follow your marker outside to liberate the Henbane River Region.
The game may also tell you that the River Armed Convoy mission has completed when it hasn't, or the mission may actually complete, but the boats are still there.
Also, if you didn't officially get it from Virgil earlier, the War on Drugs mission becomes available.