Kinda silly how easy it is to miss, seeing as it's so easy to find.
I only found how to get up there by climing on top of that big boat across the road (where the small bandit camp is), and looking around with my sniper rifle. Good way to clean out his skags too, before going over to fight him.
Also i think there's a challenge to kill him without killing his skags? If that's true, climb the boat and snipe away. Perfect spot.