Although Gyan was an MT, he was still completely defeated under Sheyan's unrelenting attacks, mainly because Evil Sword Apophis was simply too absurdly powerful.
This time, Sheyan did not perform the second half of 'Withering Impact' after sending Gyan up into the air because his main focus was still on the woman named Artemis. Instead, he waited for Gyan to fall, then swung his sword with a face full of rage. He used the 'Extinction' ability to hack off Gyan's right foot.
This way, although the destructive power of the second half of the 'Withering Impact' did not materialise, Sheyan would not have to bear the subsequent side effect of not being able to move for a long time. He would thus have enough mobility to pursue after Artemis.