Question

How did some formerly enslaved African Americans in the South respond to advancing Union forces?

Answer 1

They served as scouts and spies for the union. They also enlisted in the union.

Answer 2

The tactics that The former enslaved African Americans use to  respond to the union force's attack were:  they served as scouts and spies for the Union

Due to the shortage in forces, the southern states at that time forces the slaves to become a part of their military (they would be sent to dangerous area of the war).

Since the enslaved African American knew that the union come to free them, they leverage their position in the army to leak crucial information that help the union won the war.