on the online version it is a group kill so although u might not see any damage being done the damage u do is recorded and uploaded to the server if u want to kill him on ur own i would suggest switching ur game to offline and attack the
glowing spotson his body those are the weak spots i beleve that there are 2 on head 2 on each leg 1 on belly one on each wing and one on tail i could be wrong about numbers but im pretty sure about them