The leader of the experimental subjects clashed with Stephen, while Ethan faced off with two fiends at the same time. Lynn was nowhere to be seen because she had already activated her invisibility.
After more than a month of practice, she had extended the duration of her skill to forty minutes. That was the longest she could turn invisible at a given time. If her ability expired on the time limit, she wouldn't be able to use it again for twenty minutes.
If she deactivated her invisibility before thirty minutes was up, she wouldn't get the twenty minute penalty. Once it surpassed a certain threshold there must be a cool down time for her body to adjust, and for her ability to take effect once again.
The children were dealing with twenty experimental subjects. However, Ciel knew that their main target was him. Since that was the case, he flew close to the ground in order to lure the other fiends towards him.