Ramayana
अहत्वा तांश्च दुर्धर्षान्राक्षसान्कामरूपिणः । न शक्यो रावणो हन्तुं येन सा मैथिली हृता ॥ ४-३५-१६
'It is not possible to kill Ravana who has stolen Sita without killing those unassailable demons who can assume any form at their will. ॥ 4-35-16॥
english translation
ahatvA tAMzca durdharSAnrAkSasAnkAmarUpiNaH । na zakyo rAvaNo hantuM yena sA maithilI hRtA ॥ 4-35-16
hk transliteration by Sanscript