I've probably done ~ 30-40 runs in Scylla's Grove and probably 10 tries with Dexi and have yet to see him. Patience.
That's the problem for me. I'm not a patient man. This is not as simple as farming a boss either. It's like trying to get Verm to spawn from the Varkids. Some people say it took them hours before he spawned. I don't have that kinda patience.
Then dont play loot games. Or just ask for dupes from the get-go.