I did the third part of the quest for my 45 Mage, (I know this quest is for a Cleric or Druid) but am doing for faction as well, in about 1 hour. You can get Caustic Fluid in Unrest as well. Mistmoore has been changed back since the Halloween Event occurred so no longer a 60+ zone anymore. Rancorous Ghasts spawn betw game time of 6 PM - 6 AM and I had 5 in of those 12 hour sessions to drop 3 Putrescent Hearts. I already had 1 so I was finished with that part with a 46 Druid helping with a 123 tracking ability. Got that part done, after figuring out what to do, in about 2 hours.
Finnyiniel