How to ninja loot Mop rares

As you may know there is a kind of mop rares that during the fight vanish and then repop. ( This for example Nessos the Oracle )

2 days ago i found a ppl of the other faction doing one of these rares, so i helped him and i found out that: if you attack the mob first when he comes out from the last vanish you can loot him.
I was not sure so i tried 2-3 times and it seems to work.

What i'm not sure of:

• Don't know if this works when the people leaves dot on the mob when he vanished 
• Don't know if this works with people of your faction.

PS: first post here i put this in elite section cause of i would not see 20 ppls on a rare trying to steal it