Knowing Brutus was a peak A+ rank hunter, Valery already knew she wouldn't be able to harm him using normal attacks, especially because of his powerful defensive skills.
She was losing mana at a very fast speed because of ice Reaper Form, and although she could replenish mana using a potion, the burden of using the full power of her physique was still too much for her. So instead of wasting her energy in a meaningless fight, she decided to use her strongest attack while he was still distracted.
Taking advantage of dust and debris that were blocking everyone's vision, Valery activated her unique skill 'Frozen World' that she used during the practical exam of the academy.
But this time instead of directly casting the skill, she used her spear as a medium to cast her skill. The spear was actually an A+ rank artefact that could increase the power of all of her ice type skills by 100%