Indrajit eliminated

 

Vibhishana comes to Lakshmana and beholds Sri Rama having fallen into a swoon and lying in the lap of Lakshmana. Lakshmana informs Vibhishana that on hearing the that Seetha was killed by Indrajit, Sri Rama fell into a swoon. Explaining Sri Rama the secret of conjuring trick practiced by Indrajit in killing an illusory image of Seetha, Vibhishana assures him of Seetha being still alive and urges him to send Lakshmana with an army to the sanctuary of Nikumbhila.

Vibhishana suggests Sri Rama to send Lakshmana to kill Indrajit, even before Indrajit completes the sacrificial fire in the sanctuary of Nikumbhila. Sri Rama then asks Lakshmana to proceed with the battle along with armies of Sugreeva, Sri Hanuman, Jambavan and Vibhishana. Lakshmana proceeds to Nikumbhila along with Sri Hanuma, Vibhishana and Jambavan to kill Indrajit. Lakshmana accompanied by Vibhishana, Angada and Sri Hanuma penetrates army of the enemies at the sanctuary of Nikumbhila.

In a terrific fight that ensued, Lakshmana and Indrajit, who were engaged in releasing several streams of arrows, fought like two lions. Lakshmana the foremost among humans and Indrajit the foremost among demons fought with rejoice.

That fierce battle went on for a longer period due to exchange of many ASTRAs presided by various deities like Varuna, Agni, Maheswara, etc.

Finally fitting a missile presided over by Indra to his bow and drawing it, Lakshmana discharged it at Indrajit, after making the following prayer:

धर्मात्मा सत्यसन्धश्च रामो दाशरथिर्यदि ||
पौरुषे चाप्रतिद्वन्द्वस्तदेनं जहि रावणिम् |  

(Yuddha Kanda - Sarga 90 -  Sloka 71)

“O my dear arrow! If Rama the son of Dasaratha has set his mind on virtue (Dharma), adhered to TRUTH and is second to none in his prowess, destroy this Indrajit.” 

That large head of Indrajit, separated from his trunk and moistened with blood, was seen on the ground, shining like gold.

We have to understand that the knowledge of Lakshmana in divine weapons did not kill the great warrior Indrajit.  

It was the ascetic power of Sri Rama consisting of virtue (Dharma - धर्मात्मा), adherence to TRUTH (सत्यसन्धश्च) and is second to none in his prowess (or) not to be vanquished- अप्रतिद्वन्द्व: पौरुषे) that eliminated the Indrajit.  

Lakshmana was  only instrumental.