|Head to Seed Ranch (southwest Holland Valley) or Wingman Story Mission||400 RP, Speeder Outfit|
Note: Due to the size of this outpost, the number of alarms (3) and all of the cultists, this is the most difficult outpost in the Valley. You'll want at least the Sneaky Sprint Perk and possibly also the Takedown Mastery Perk.
You'll want to begin by scouting the area out, there's a lot of cultists here and the last thing you want to do is go in unprepared. Take the time to go to a tower just north of the ranch and grapple up, this is an excellent location for a stakeout!
Once you've tagged all 12 of the cultists here (three of which are snipers) and the three alarms, take the southeast zipline back down and arrive behind the estate, this is the best place to begin clearing out the outpost.
From the southeast of the ranch, a little way south of the two cultists standing together, you should see the first alarm. Wait for the close to be clear and move to the porch just behind it. While staying hidden behind the fence here, pop up and put a bullet into it with the silenced handgun.
Turn back around from here and start circling around back, your first target is patrolling here, wait for him to approach and dispatch him (be patient while you wait for him, he does have a large patrol area). Continue along until you reach a cultist sitting on an elevated platform, sneak up on him from behind the rocks and take him down from behind the boxes.
Sneak towards the back of the house and wait for the next patrol to work its way around back before eliminating him, taking care not to be spotted by the cultist on the balcony.
Tip: With the frequent patrols here, you'll have to make good use of moving any bodies out of the open to avoid wasting all your hard work.
Head back around the side of the house here and position yourself between the two structures. You'll see a Marksman cultist on the balcony to the left and another to the right near the second alarm. Wait below the stairs of the Marksman cultist and when the right cultist turns his back, quickly takedown the marksman and grab his body before retreating back below the stairs and throwing it behind the house (if you didn't grab the body here, the other cultist will notice it and head over to inspect, take him down on his journey there).
Go over to the stairs on your right and wait for the cultist to turn his back before taking him down and carefully putting a silenced bullet into the second alarm here. Go back behind the left of the two houses again and take the ladder up. Carefully ascend the roof ahead and then work your way towards the watchtower and climb over the balcony. You'll be able to sneak up on the cultist here without any problems before looking over towards the last alarm that you marked, one more bullet puts all of the alarms out of commision.
Make your way off the roof the way you came and once more return to the passageway between the two houses, there will be a cultist here who is waiting to be taken out. The next two cultists are just in front of the watchtower, wait for the one close to your position and take him out while the one at back isn't looking before heading over dispatching him.
Just three more cultists remain, the first of your targets being the lone cultist on his own, position yourself with your back to the house and take him out. Quickly pickup the body and take it with you back to the house, the Marksman cultist patrols here too. Drop the body and takedown the Marksman when he turns around before proceeding to the last cultist and liberating the hardest outpost in Holland Valley, congratulations!