Zach was confident because he could entrust this matter to Diana. Had he been alone, he might not have been as confident in using the same strategy. But Diana had been practicing with a bow to shoot ever since her rebellious days when she used to run away from home to improve her combat skills. She had not only honed her skills in a hostile environment but had also become an accomplished archer before shifting over to the hot weapons that Zach introduced her to. But expecting such a situation, Diana had never stopped practicing her archery despite knowing that better range weapons were available to her.
Since Zach didn't tell her anything about the contents of the bottles, Diana could only take a guess as to what the bottles contained inside them. She believed that it must be some kind of a strong poison that could help them in battle. They didn't have the time for her to bring up such an important matter at this crucial time.