Riker kept gradually upping the phaser, it was pretty obvious there was no setting that would have stopped her but not killed her. Maybe repeated stuns would have stopped her, but she was lunging forward at him desperately, she only had to touch him to kill him, and you can't risk the victim's life to save the life of the attacker.
A one second burst had Yuta stumbling backwards, barely able to stand. Hit her with that for ten seconds and she'd be on the floor in agony. Better than killing her.