What is the legal ruling on the phrase: "Only He remains as He is," which is intended to mean that God alone is He whose state does not change?

There is no issue with applying the terms "eternal" (dawam) and "perfect" (kamal) to Allah Almighty and denying them to all else. This is supported by His (the Most High) saying: "Everyone upon it will perish, And there will remain the Face of your Lord, Owner of Majesty and Honor," and His (the Most High) saying: "Everything will perish except His Face."