Once you've settled into the Holland Valley, and are somewhere between 1 and 2 resistance points, you'll get a long radio call from John, that ends with you being Marked.
Enemies will converge from all sides. As the saying goes, you can run but you'll only die tired. Save yourself the trouble and let them catch you (don't worry, it's all part of the story).
There's a little cutscene in which the Father plays your unexpected savior, then you're tossed into the back of a prisoner van, which crashes. You'll be rescued by Father Jerome of Fall's End.
He'll ask you to help him secure an Eden checkpoint. This marks the start of the mission proper.
You're not going to take these guys by surprise, so charge right in with your assault rifle. A good plan is-
(a) explosives to kill/confuse the ground forces,
(b) shoot the snipers.
(c) mop up whoever's left.
Do remember to grab a sniper rifle from the upper level before proceeding.
Talk to Jerome, then follow the path up to free more prisoners.
Past that is a section where a bridge is being covered by mortar fire. Because why not.
Use the zip line up in the rocks to make a quick approach between bombardments, then proceed toward your goal keeping under the bridges, and low in general.
Approach the last mesa from the right to get a drop on Merle's guards.
Free Merle, then hold out for two minutes until the chopper arrives. You can use the mortar, but it's almost easier to stick to the sniper rifle. There seem to be only two avenues of attack for the enemy horde- over the bridge, and up the cliff to your right (west).
Hang close to Merle and keep the hordes at bay until you can board the chopper (also on the west cliff).
When the chopper lands, your mission is complete.
You'll regain access to all your old stuff & companions, and more Armed Convoys will appear in the valley.