Like spoderman said, explosions might b the issue but not all the time, it's either that or the rocks, how? Well I asked myself the same question until I experienced it first hand. As soon as he gets out he either goes left or right and we killed him two times in a row next to the rock on the right where he was hiding with snipers only and when our sword bearer got there it immediately disappeared, the exact same thing happened the second time so we decided not to kill him next the any rock and had no issues after that.