Usually ἐλελεῦ is an onomatopoetic expression of pain or sorrow. The only place I can find it used as a war cry is in Aristophanes' Birds. Generally the verb for the war cry is ἀλαλάζω. While reconstructing the sound of ancient speech is always a challenge, Hesiod says that the Athenians adopted the cry to imitate their patron goddess' sacred bird, the owl.