here is a break down of the quest:

the essences drop from the dark broodlings- these can be found in 'terror wing pass' all the way to the east

you need to find a way to use this gun in melee and not let them hit you- Im a hunter so I just sent my pet in 1st, locks and do the same... priests can sheild... other classes might have to do this with a friend (or time your distance right and start off using the gun without agroing)... however you do it, just hit him with the gun at some point before he's dead- once you do a purple cloud will surround your broodling..
once hes dead a small piller appears- loot it and it has the essence in it

100% drop rate so long as you hit em with the gun... be careful in the pass though, lots of 52-53 eilites walking/flying around