@ David Beckham Ravan was granted some boons by Brahma that he would never be defeated or killed by any of powerful Dev, Danav, animal or any celestial being. Regarding Manav(Human), Ravan was confident that he would take care of any manav himself. It is said that Vishnu then himself took the form of Manav (Rama) to get rid of Ravan. Since Rama had barred himself from his Kingdom & his army for 14 years to keep his fathers promise, he had to fight Ravan with the minimal resources he had in the forest. Hanuman, Sugreev & his Vanar Sena was there to help him out. This was just to prove the point(learning point indeed) that the victory belongs to one who has Satya(truth) & Dharm on his side.
