Complex sentence: The French advance drove in the British skirmishers and reached the sunken road, as they did so, Pack's men stood up, formed into a 4 deep line formation for fear of the French cavalry, advanced and opened fire.
Split into simpler sentences: The French advance drove in the British skirmishers and reached the sunken road. As they did so, Pack's men stood up, formed into a four deep line formation for fear of the French cavalry, advanced, and opened fire.