Complex sentence: The Americans were outnumbered and being hit with armour and heavy weapons, but the reverse incline gave them the ability to direct all their firepower simultaneously at the enemy units as they appeared one-by-one over the top of the hill.
Split into simpler sentences: The Americans were outnumbered and being hit with armour and heavy weapons. But the reverse incline gave them the ability to direct all their firepower simultaneously at the enemy units as they appeared one-by-one over the top of the hill.