Like many anime, Black Clover uses plot armor to keep characters from dying too soon. Sometimes, this tool was misused, and it brought more harm than good to the series. Lots of times in the show, plot armor was used in a more underwhelming way and made the story look cheap.
William is a very powerful mage, but he's not had enough chances to show what he can really do. The fight against Zenon should have been the perfect opportunity to display his full strength. However, he chose to save his squad members instead of fighting with all his might.
Saving his comrades weakened William's focus and dispersed his magic power. Protecting his squad members was a noble thing to do, but if he had given it his all, he might have been able to take the fight elsewhere and keep his subordinates away from harm.