It seemed that Lucyna wasn't able to steal more mana than she could use. Perhaps another one hundred points in magic will do the trick, but if she just focuses on support, then she can save her energy.
"Although it is temporary, you can steal the target's endurance as well, right?" Billy asked. "Just do that on the enemies that attack our back. That way, you will save mana, and so will the others."
"I never mentioned that I could, so how do you know that?" Lucyna asked.
"It was just a guess," Billy replied while shrugging.
"... I suppose that if we fight all on the frontlines, things will get out of hand eventually," Lucyna said. "I will be sure to offer you some support from the backline."