Once you’ve reached Resistance Level 1 in John’s region, he’ll send out a capture party to you. You won’t be able to escape them since one hit from their bliss-induced bullets will result in your capture and automatically starting this mission.
After the cutscene, you’ll have to help The Resistance fighters in taking over the checkpoint. There’s a lot of peggies here so find cover immediately and pick them off one by one. The enemies will take cover and clump together as well so use explosives, grenades, or dynamites to quickly flush them out and take several of them out.
Once the checkpoint is cleared, continue along the dirt road until you raech a bridge. There’s a mortar in the rocky outcrops on the other side that will continue raining shells so don’t cross the bridge in the open. Instead, run through the left side of the bridge then use the zip line (don’t cross the hanging bridge) to reach the other side quickly.
Carefully climb the rocks until you reach the mortar. Kill the peggy operating it then free Merle who’s lying nearby. Free Merle then hold your ground against a peggy Assault. You can use the mortar nearby to bomb incoming peggies but you can’t rely on it alone since the peggies can overrun you quite easily.
Keep killing them until the timer expires. Once the chopper arrives, quickly hop inside and shoot the cultists when you can. The chopper will leave once Merle hops in as well. After arriving to a safe location, the mission will be complete.