William intended to do so, in case of everyone else coming at him, not just one enemy.
As he already spotted where the attack was coming from, he was able to perfectly evade it. Yet all he did was to take out a sword, moving in a sluggish way to the side, ending up clashing with the incoming enemy, blocking most of the attack, ending up suffering a little wound at his flank.
It wasn't that deep, but William added a last touch of making his body struggle to stabilise himself for a few moments, before he finally leant on one knee.
He planted the sword in the ground, supported himself against it with one hand, while holding the wound he suffered with the other one. "I swear to kill you!" he coldly said, in a threatening way that didn't carry any weight.
"Hahahaha! Just try and stand up straight, you moron, hahahaha!" seeing this made the leader of the dark master group laugh wholeheartedly, mocking William.