This boss only spawns during the story quest. You have to loot the wagon (cart) next to the gun and the defend it for 3min. There is repeatable faction quest that want you to kill him, but it's not possible to do that quest as you can summon this boss only during the quest. Classic AGS :)
How does this SPAWN anyone know????????
Destroying all city-slickers and their fancy German cars.
You can repeatedly do the event if you stay in the village it keeps popping over and over. He dropped me ''Feedsack Britches of the Sentry'' 3 times out of 3. Seems like a Guaranteed drop. Not sure if the Sprout of Life staff can really drop though. Will keep killing him a few more times and see