Complex sentence: When attacking an enemy position equipped with anti-tank guns and artillery, as occurred near Mogilev and Rogachev, because of its high superstructure that presents a good target for artillery and anti-tank guns, the "Panzerjäger" is destroyed before it can get into action.
Split into simpler sentences: When attacking an enemy position equipped with anti-tank guns and artillery, namely near Mogilev and Rogachev, its rather tall superstructure presented a target for artillery and anti-tank guns. Thus the "Panzerjäger" is destroyed before it can get into action.