By what factor brighter was the supernova ASASSN-15lh found to be compared to the entire Milky Way galaxy at its peak?

Answer

Roughly 50 times brighter

The specific event ASASSN-15lh was observed to have a peak luminosity approximately 50 times greater than the combined light output of the Milky Way galaxy.

By what factor brighter was the supernova ASASSN-15lh found to be compared to the entire Milky Way galaxy at its peak?
galaxyastronomystarssupernovabrightness