Blow Their Mine (Story Mission)
After completing Razing the Steaks, speak to Zip again to begin this story mission.
This one ends with one of the hardest fights in an optional mission, so make sure your supplies are topped off and your'e carrying your favorite weapons before heading out. In particular, you'll probably want a good assault rifle, and maybe a sniper rifle or LMG to back it up. When ready, set off for the gold mine.
It is possible to stealth-apply the explosives to all four sites by coming in along railroad tracks from the west. However, this whole place is about to go battle-royale, so a better strategy is to clear the site.
Before setting all the charges, do take note of how the four sites connect to each other- the railway tracks for the upper two, the building walkways inside, and the zipline.
Also note the mounted gun out back (south) of the building with most of the charge sites.
When you feel comfortable with the area, go ahead and set your charges. You now have to defend each of the four explosives from wave after wave of incoming cultists. Judicious use of a rocket launcher can stave off the first wave a bit, but enemies will soon be coming in continuously from all sides.
The mounted gun itself isn't as useful as you might like, but that's a good area to hang out it- easily defensible, and with quick access to any of the four sites. If a cultist does reach a bomb, you'll see special wire-cutter icon, and you'll have about forty seconds to stop them.
Defend all bombs for the full four minutes, and Zip will give you a plot marker by the front gate. Reach the marker to watch the explosions, and complete the mission.
Note that completing this mission also gives you a more efficient recipe for dynamite.