We need to understand Ramayana in the language of Internal Ramayana taking place in the body everyday in which Ram represents consciousness, Sita the body and Lakshman, determination or mind with an aim. Here Meghnaad means Rajas or Rajoguna which can be killed easily by controlling the mind. Rajoguna activates anger, cynicism, aggression etc. Once you control the mind, you can kill your aggressive behavior.
Lakshman killing Meghnaad means that for killing of Rajoguna you do not need the help of the consciousness or the Rama.