Fortunately it did not take Allen that much time to kill the monster around that place. As he already became a level 50, naturally everyone of those Monsters wasn't even able to do anything to him. First of all he was still present in the inner area and every Monster present here most likely around level 20 or level 30. So naturally it was very easy for Allen to kill those Monsters.
But you can say that he did not want to kill that much Monster because killing them wasn't helping him in anything and he just wanted to go to the core area as soon as he could. So after killing those black Scorpions he once again began to advance using his movement technique and with that his speed already increased.
Like this he began to advance and if his guess was right then you will reach the core area after 4 to 5 hours later. Sometime later he decided to rest for sometime because he was just continuously running like this and with that he already began to feel hungry.