When Yuki managed to counter most of the attacks which were getting directed at him by the goblins, he was still left with an army of them which all continued to attempt to kill him even though it was impossible for them to do so due to the power gap between Yuki and them. Even though Yuki knew that he could slaughter them all in the matter of seconds, he chose to extend the duration of the battle for the simple reason of him wanting to improve his technique. He knew that pure strength would eventually not be enough to take down his opponents. At some point, he will have to face someone who has equal or more strength than him. At that point, all he would be able to rely on was his skill. Due to him losing most of his fighting techniques during the 1 year break he took from dungeon raiding, Yuki needed to make up for it by making his journey harder on the early stages of the dungeon.