Complex sentence: Holmes is also one of only five boxers--along with Joe Frazier, Ken Norton, Leon Spinks and Trevor Berbick--to defeat Muhammad Ali, but is the only one to have ever stopped Ali.
Split into simpler sentences: Holmes is also one of only five boxers--along with Joe Frazier, Ken Norton, Leon Spinks and Trevor Berbick--to defeat Muhammad Ali. He is also the only one to have ever stopped Ali.