As of writing this answer, the current world population is over 7,300,000,000 people. 20% of this number comes to 1,460,000,000 people, which is about 4.5 times as large as the United States.
If we wished to reduce our population by this amount it would require a catastrophic amount of energy or force on the scale of tsunamis large enough to cover the entire Chinese coastline. Plagues, illnesses, and other diseases would not be efficient enough as they require spreading, incubation in their victims, and for the person to succumb to the disease. In order to lower the world population so quickly, large scale bombing could be used on high density cities such as Delhi, New York, Hong Kong, and any other city with more than a million people. The largest city by population, Tokyo, Japan, only has 13 million people, so every large scale major city around the globe would have to be targeted.
A large amount of seismic activity could also help to reduce the overall population. If the Ring of Fire (the volcanoes in the Pacific Ocean) were to erupt and cause large enough earthquakes, the resulting tidal waves could easily wipe out much of the Pacific Islands and many coastal towns of Eastern Asia. Beyond these two methods I'm not sure there are any other ways that could work as quickly or to the same magnitude as these options. Hope this helps!