Complex sentence: Cumberland's force mounted a devastating bombardment on the Jacobite army which was stationery and exposed on the open terrain; up to a third of Charles Edward's force were dispersed or made casualties, (including a groom decapitated while holding the prince's horse).
Split into simpler sentences: Cumberland's force mounted a devastating bombardment on the Jacobite army. Stationery and exposed on the open terrain up to a third of Charles' men were dispersed or made casualties, (including a groom decapitated while holding Charles' horse).