Reincarnation Won't Drop

I've been trying to finish Reincarnation with my friend, however every single time we kill him, it only drops for her. We've killed him at least fifteen times now, and I've made some of the killing blows as well, but it only ever drops for her so I am unable to finish the quest. Help?

 

In order for a mob to drop an item for you, you must do at minimum 15% of its health in damage. Try killing him again with you dealing more damage during the fight.

 

I'm pretty sure I have done at least that much damage.

 

Try again I suppose. Bob has 180k health if I remember correctly (might have changed in the most recent update), so you'll need to do at least 27k. It'll drop when you do enough damage.

 

May I ask what weapon you are using?