Return of Reckoning

I've been hunting Sheen Gloomleaf on my alts for the acheivement "Death in Green and Brown" and im currently on my fourth character and for some reason this mob is no longer spawning. I've been camping it for well over a day now. To be fair, im not sure what the spawn time is on him, but I know I did this achievement twice in 2 days so Id assume its not on a 48 hour timer.
Thanks

*edit* if found him, just in a different location. after he killed me it looked like he ran back to his normal spot.

Thank you for your sacrifice.