I don't know shit about how that battle went, but I can assume it wasn't against eight feet tall daemon-powered enhanced hulking supersoldiers in power armour. Compared to their sheer bulk and strength, the shields buckled. While the men fought with bravery, in a contest of pure strength they wouldn't last very long. Luckily, they wouldn't need to. With the mysterious assault from the rear, the Chaos troops had begun dwindling to a point where retreat was necessary. The sorcerer at the center of the formation opened a warp portal, allowing several of them to escape, leaving a few last heretics to be slaughtered.