 अहत्वा तांश्च दुर्धर्षान्राक्षसान्कामरूपिणः।न शक्यो रावणो हन्तुं येन सा मैथिली हृता।। 
 It is not possible to kill Ravana who has stolen Sita without killing those unassailable demons who can assume any form at their will. 