Warden Stillwater not spawning to finish the quest.
Same. Warden Stillwater will not spawn for the final quest line Lawn of the Dead “Warden Stillwater Defeated”.
Bug is still ongoing just tried to do it a few times Warden Stillwater doesn’t show up just endless waves.
Still bugged, can’t get Stillwater to spawn 
So I was running into this issue as well before today at Level 59 (no Chromie Time).
But today I was passing by while Chromie Time was enabled and I decided to give it a shot. Stillwater spawned as he should and I was able to complete the quest.
I know its not a solution for those LvL 60+, but its a work around for those below.