“Do not be afraid because of them.” Fear would be understandable given the size of the alliance against Israel.
“I will make them all slain ones before Israel.” The Israelites had to do the fighting, but the victory was due to God’s work in the situation (cp. Josh. 11:8). The Hebrew text uses the word “give,” and reads more like “I will give them slain before Israel.” God gives the victory, which is why it is so vital for believers to serve and obey God if they want to be truly successful.
“You are to hamstring their horses.” God knew that the human tendency of a person with horses and chariots was to trust them rather than Him, so He commanded the horses be hamstrung and the chariots burned (cp. Ps. 20:7).