The arrow was one of Rand's trump cards.
Penetrating Ice Killing Arrow.
He learned this skill from the Bow Tower when he became an Inner Disciple three years ago. In fact, if one were to ask him, he would say that skill was one of his strongest skills.
The skill taps into his core ice and forms an arrow from it.
This core ice is the strongest form of his ice element, and the arrow is formed from it. While he wouldn't say it was easy, he also never expected Unity to survive that.
An arrow formed from the core ice is something nobody can easily defend against, especially someone as weak as Unity, who wasn't even a Root Saint yet.
Just like Kent, she was also at the Level 8 Root Grandmaster stage. But when the arrow came speeding toward her, she remained calm and retrieved a single arrow from her quiver.
She nocked it into her bow and imbued it with her energy. She was still not using her lightning element, so it was a pure brute attack.