For Zhou Tian, the invasion was now just a matter of tying up loose ends. Given his current fire resistance attribute, Zhou Tian did not consider the Monster's Boss to pose any threat to him. In other words, if Zhou Tian were to take action against that Monster, he was quite confident that he would surely defeat it.
Thus, Zhou Tian himself was already capable of defeating that Monster, and while Bella's presence by his side was not entirely without effect, to speak frankly, the help she could offer at that moment was indeed quite limited. If he were to keep her by his side, it would undoubtedly be a waste of combat power.
If it weren't for the actions of the nearby Monsters, Zhou Tian probably wouldn't have said anything more; after all, Bella was a Domain powerhouse in her own right. Even if she couldn't be of much help, her mere presence could certainly boost their morale and add to their already solid winning odds.