Originally Posted by JuSt BLaZe
In order to make the Leviathan's body easily dissapear you need to do either the mission X MARKS THE SPOT or TREASURE OF THE SANDS and DO NOT TURN THEM IN. Then leave the lair and always get there from Oasis. Simple as that!
I turned both mine in fine and the body is never there. I always re-enter where you come out at the back of Oasis.