mostly dialogue between Sansui and Ran. Ran makes a small adjustment and haughtily proclaims that she has overcome him (technically in speed alone, she does), she tries, he overcomes her and berates her lack of self-control. That is the pattern the fight takes; he sets up a scenario, she rushes in without defense, he counters and explains why she failed. The importance of the "fight" isn't the combat, but rather if she can prove that she learn self-control before he decides to execute her. He explicitly says it isn't a fight. In a nutshell, Sansui's test is pretty much the antagonist here, the hurdle Ran must overcome.
Also, he doesn't need to get serious when fighting her because he is still able to read Ran's movements due to himself being extremely disciplined. He is supposed to be one-sidedly mentally beating the proud 'might makes right' ideology out of her to the point where the audience feels sorry for her. It ends with Ran in tears after he successfully intimidates her with the reality of the situation that he will flat-out kill her with no chance of survival if she does not overcome her berserk state.
There is a reason he is not the harem protagonist of this series.