Throw a smoke pellet at Harley to disorientate her
From Create Your Own Story
Location: Wayne Enterprises R&D facility East Wing
Batman quickly grabbed a smoke pellet from his belt, and threw it at Harley, disorientating her. She yelled:
"Wrong move, B-man."
Since Harley never moved her gun away from the scientist, she knew that she could just pull the trigger, even though she couldn't see him.
A gunshot was heard, and as soon as the smoke cleared up, Batman saw the scientist dead on the ground.
"Should've been more careful, Bat-brain."
Game Over