Complex sentence: Some of his toughest fights had been with Jarvis "Death" Oliver, the Croydon Drover, Fry "Bill" Warr, Isaac Perrins and Michael Ryan, whom he met twice.
Split into simpler sentences: Some of his toughest fights were against Jarvis, "Death" Oliver, the Croydon Drover, Fry. Bill Warr, Isaac Perrins and Michael Ryan, whom he met twice.