As soon as Satoshi disappeared from view, Emma prepared a couple of portals around her since there was a good chance Shiroi would launch a surprise attack. She was right on the money. In an instant, Shiroi sent out a couple of ice spikes her way before she started charging.
Emma jumped into one portal, trying to flank Shiroi who had attacked her without a signal. That's the only thing she could do since she was severely disadvantaged against Shiroi. While Shiroi's superpower can be used for offense and support, her superpower was purely support type. That's why even if she could get near Shiroi, she won't be able to attack without getting hurt. The only thing she could do was keep her at bay.
However, Emma knew that that was not what Satoshi wanted. She was aware of how powerless she was against offensive types, but if she remained that way, then she stood no chance against her would-be opponents on the school's testing day.