Adrian clearly saw the damaged tail suddenly retreat into the deep sea, but he tried to pull it up. He used a powerful gravitational pull, but it seems that the strength of the tail is greater than he anticipated. He could not perform such a heavy task with the remaining mana that he had. He had to accept that this was the only result of the fight.
They were able to destroy the tail, but it retreated when it dropped to 20% health which means that there are only two more Leviathan exposed parts that needs to be destroyed. Adrian thought that they would actually cut the tail itself, but they only need to destroy it until the Leviathan's tail retreats. This news also reached the players attacking the other body parts of the Leviathan as they knew that it is incredibly difficult to destroy.