Hezekiah said that with the king of Assyria was only an arm of what?

Answer: Flesh

Hezekiah encouraged Judah: "With him is only the arm of flesh, but with us is the LORD our God to help us."

“With him is only the arm of flesh, but with us is the LORD our God to help us and to fight our battles.” So the people were strengthened by the words of Hezekiah king of Judah.”— 2 Chronicles 32:8 (BSB)