durātmana: of the evil minded, tasya rāvaṇasya that Ravana's, vadhē in the matter of destruction, upāya: strategies, ka: what?, yam by which ever strategies, āsthāya after adopting, ṛṣikaṇṭakam thorn by the side of sages(causing sufferings to sages), tam him, aham I, nihanyām I shall kill.
"What strategies should be adopted in the matter of destruction of that evil-minded Ravana who is a thorn to the sages?".