Complex sentence: Personally leading the Celtic charge, Aneorestus and his forces nearly broke through the Roman lines when the Boii and their allies were routed the Romans on the opposite wing of the battle flanked the Gaesatae and Insubre warbands.
Split into simpler sentences: Personally leading the Celtic charge, Aneorestus and his forces nearly broke through the Roman lines when the Boii and their allies were routed. Following the victory, the Romans on the that wing of the battle flanked the Gaesatae and Insubre warbands.