They had a stare-off! Anna is situated on the right and Vincent by the left. Since the village was short of weapons, they decided it was going to be a fist contest.
That works out well in Vincent's favour. Anna's expertise is the sword, a weapon. Imagine having her use one in a friendly fire with an ordinary!
'I shouldn't think such frightening thoughts'.
He had ordered Anna not to use any mage abilities and that included the raw strength that came with being a mage. She was to tone it all down to how she was when she was once a human.
It was not a difficult order for Anna to comply with. As someone who had experienced the near epitome of what it feels like as a human, she was able to easily control her physical ability to reverse to that of a human.
Reverse in this case meant she suppressed her mage physique with sheer will alone.
Anna can always have it resurface when the need arises.