The best thing to do is nothing, as, since they appear within your lines, support will hurt you as well. A thing which you could do is buy a SpecOps, or similar, but he'll probably be pinned and die as soon as he comes on. So just sit, pray, and hopefully your men will kill them all. Often a Game Ender for the rookie.
How Do I Deal With This?
If your men were entrenched on the left or right hand side of the map, tell them to avoid any CQC as possible, because in a time like this, you need all your men to fire their guns. Brandenburger men have the dreaded MG 26, a very accurate weapon, making them deadly at close range.