Final answer:
Charles Grandison Finney is likely the American circuit-rider referred to, known for his involvement in the Second Great Awakening and his evangelical work in the 19th century.
Step-by-step explanation:
The famous American circuit-rider who knocked down bullies, preached at camp meetings, and served for 20 years as an itinerant plus 50 years as a presiding elder was Charles Grandison Finney. Finney, a significant figure in the Second Great Awakening, was known for leading revival meetings and promoting Evangelical Christianity during the 19th century. His work began after a religious conversion in 1821, and he became renowned for his ministry in Rochester, New York, which thrived with the development of the Erie Canal. Although many other influential evangelists like Billy Sunday and Aimee Semple McPherson made significant impacts during their time, Charles Grandison Finney is the most likely answer to this question given his long-term service and the specific roles mentioned.