You have to kill a volume of mudman > the size of the Gorge so that Xorbb becomes threatened that you are going to destroy his home.
This is either false or doesnt hold true on a server up. Either way when he spawned for us was for 3-4 days of inactivity by just looking at track and he finally spawned(after days of perma chain tag team killing the zone inside out.
Heh. Though I love the fun of the fake conspiracy theory... He is simply 1% on a slow spawn... so way rare.
Took us something like 4-5 days to pop him. Then he popped again like ~90 minutes later, dropping a Slime Staff of Winning both times. Fun times.