Getting hurt was a grave matter to Ye Zisu and the rest, but Achilles and Lie Xin did not really take it seriously. Things were fine as long as nobody died. Getting hurt was nothing out of the ordinary.
Achilles had gained a strong interest in Lie Xin's breakthrough. This was the true power of the Lie Flame Technique - it completely disregarded all of the usual rules. The restrictions of Tita Star were meaningless to her. In fact, the special characteristics of Tita Star seemed to serve as firewood for Lie Xin's Lie Flame Technique, igniting her powers instead of inhibiting them.
Wood gave rise to fire - this was an axiom of the five elements. The theory was simple but putting it into practice was very difficult. The power from such an interaction could only be harnessed once one's strength reached a certain level. Lie Xin's attack demonstrated that she indeed possessed such skill.