Decisiveness
The timing had to be just impeccable, or else the Grand Elder would've noticed her and alerted the entire Tripartite Alliance which would be detrimental to them, maybe even leading to their deaths.
She could sense that the Grand Elder was a Peak-Level Law Dominion Stage Expert as he didn't hide his undulations. If she openly fought, she would require more time to defeat and even more time to kill the Grand Elder.
By that time, the people she had to save would have all died by then...
Nevertheless, she sacrificed a courageous soul to fulfill the victory conditions.
Princess Isabella gazed at the woman who was shedding tears in front of her as an apologetic glint flashed past her eyes.
However, she disappeared and the next second, the other cultivators also moved.
Princess Isabella's silhouette was nowhere to be found yet the cultivators of the Arc Song Mercenaries were exploding into a mangled mess of flesh every time a breeze flew past them.