While Astrid, Rose, and Maggie watched the battle between the adventurers and mercenaries on one side of the clearing, Lucien killed many mages on the other side.
It is easy to see that the mercenaries had the numerical advantage, so Lucien didn't think twice about starting to shoot his black arrows at them.
He planned to let the two groups kill themselves while he just protects the two main leaders and Jeanne, but he couldn't do that if the mercenaries easily beat the adventurers.
Lucien was much stronger than before and was still absorbing demonic energy. His women were like him too, but it would still be dangerous to try to face large groups of powerful opponents.
Continuing to kill mercenaries at a distance while balancing the fight was the best strategy... Lucien had already spent more than half of his black arrows and killed more than 70 mages from the Magic Band.