Altashia was soon going to get consumed by Eren's sudden wind-element attack. She had already checked those incoming wind blades with her mana sense.
Furthermore, she felt that she needed to be wary of Eren's weapon set that he threw earlier. She had seen him fight. He'd always shock his opponents with his complex moves, by making them take the decisions he wanted them to take. So she didn't try to escape from his incoming attacks in a hurry. And in doing so, she fell for the same trap she wanted to avoid.
Eren's wind-element blades weren't individually that strong. They won't be able to break her defensive spell if they attack individually. But with their quantity, they could attack the same area multiple times faster than her mana defense layer could keep up.
Plus, due to her class requirement, most of her attention had gone to controlling her mana circuits and mana points. So she couldn't strengthen her mana defense manually.
Altashia was forced to show her hand.
