Partial transfigure air at wand tip into Sarin gas (which I'm sure Harry knows the chemical structure of). Heating it will help it diffuse faster. Hold breath. But not before telling Voldemort something true put possibly useless (like the special power is "love" or something). I'm fairly certain that voldemort would feel the need to respond to this, explaining why it is a stupid answer. You have to inhale to talk. After that, make for the time-turner I guess?