In this case, I believe you misread the situation. The MC chosen and stated purpose is to use his power protect his loved ones.
Sylvie told him Tess needed him to save her. There was no time to think. You may have read the next chapter by now and learned that his intervention was the best possible move. He used his body mana protection to lessen the blow, which she couldn't do any more.
Sure, stopping this important fight before finishing it is frustrating, including for the reader, but I feel like Art priorities are the right ones, in this case.