Complex sentence: Nielsen is arguably the most popular Danish prize fighter ever, and during his career he defeated a number of famous boxers-all of whom were in their late 30's and 40's and on the downslide of their careers.
Split into simpler sentences: Nielsen is arguably the most popular Danish prize fighter ever. During his career he defeated a number of famous boxers, all of whom were in the late 30's and 40's and on the downslide of their careers.