Since he is a messenger, I suggest we give him an ability called "Diplomatic Immunity". Diplomats/messengers are not allowed to be killed, because they are only there to carry a message. The Mouth of Sauron, for example, went to Erebor even though he is the enemy, but because he is just a messenger, he was not killed. I think the same ability can be applied for Narin, for a little while he can go inside the enemy base without being attacked, but he himself can not attack enemies. It's similiar to Sauron's Annathar ability.