Lianna and Leah were both breathing heavily, having given their all to heal the two heads of the arrow formation in Colin and Leah. They had given their guarantee that they wouldn't let the two tanks fall, however it proved a lot more difficult than they had anticipated.
Not only did they have to steadily provide heals, they also had to ensure that their Law of Nature would only affect their allies. It would be disastrous if their healing were to be used on the already powerful enemy.
This required intense concentration and focus to pull off. So despite the battle only being 15 minutes thus far, they had already pushed their abilities to the limits and were suffering from some mental fatigue.
Heath and Colin were also at their limit, despite receiving healing from the two. Receiving the Qi and boosts from others was not an easy thing to endure for long periods of time.