Lakshmana was unstoppable because being the avatar of supreme godhead Vishnu and he slew Indrajit by beheading him with the Indrastra.
Lakshmana was unstoppable because most celestial astras of Indrajit refused to harm Lakshmana because Lakshmana being the avatar of supreme godhead Vishnu. Lakshmana slew Indrajit by beheading him with the Indrastra.