After fighting Rein for some time, Alcusus immediately retreated when he was unable to gain any momentum even after fighting for so long. His second skill was very powerful and it could find gaps and flaws in others techniques. But against Rein, it was not very useful as he revealed almost no gap. Whatever flaws Alcusus was able to observe were intentionally left behind by Rein to lure him.
So Alcusus thought that the more he fought this human, the more danger he would be in. So after becoming sure that he had no chance of winning in a short time, he immediately pulled back. Currently demons and humans were in a stalemate but time was on the demons side. So Alcusus showed no need to take unnecessary risks.