This is more of just a tip for Mages. I have helped a few of my guildies with this quest and told a few other Mages this tip.
When you do have the Demonic Blood what the easiest thing to do is just Poly the Felblood Initiates then cast the Siphon. As soon as it gets done casting, I start casting a fireball and normally its timed just right to keep the mob on you so no one else takes it.
Rinse, repeat until the quest is finished.
If you are any class that has a good CC its possible but thats something that I'm not sure of.
I know that there are some Uber Mages who post on these message boards, and this is not directed towards you as this would be "common sense". Thanks everyone and enjoy 2.4!