I just finished my 14th Waterworks run, and yet again I did not get my robe. I have every piece of the WW gear, but the robe. Now, I have noticed that in order to get the robe you must do one thing:
Critical kill the boss.
If you can kill the boss and you manage to critical, then you are almost guaranteed to get the robe. If you don't critical you just have a (small) chance. Every time I go and the boss is killed by a critical, whoever cast the final critical spell got the robe. If we killed the boss without any criticals, no one gets the robe.
I hope this helped some of you frustrated people out there. Next time you go try to go with people who already have their robes and are just going for other bits of gear/amulets and/or snacks. In the final boss battle, have them each focus on one of Sylvester's minions, and once they kill them, just have them boost whenever they can (so as not to avoid a super bone dragon when you attack and don't actually critical kill him). As they boost you, just spam spectral blast or some other low attack spell in your deck. Eventually you will critical kill him. Then you'll most likely get your robe.
I have not actually tried this strategy because it just dawned on me that everyone who critical kills gets their robe.
If anyone wants to argue with this theory, go ahead, but remember I have made it clear that this is an unconfirmed theory. (Although it is backed up by my experiences).
Good luck guys!