I hated them and they only give 10xp. The best way i did was to do sneak attacks on them. ED-E helps a lot since nightkin have high sneak. Otherwise send in your companions first and then stay in the back and finish them off.
I'm assuming you're doing that mission for the ghouls at the rocket site.
Anyway, the best way is to just use a 9mm and shoot for one of the legs and keep your distance. The best place to do this is to lure them in the first room (I think there were 3 controller stations lined up in the middle) where you dealt with the first Nightkin. Once one of the legs get crippled, they'll walk pretty slow so just keep your distance and use VATs to aim for the head until its dead. Keep doing it for the other 5-6 Nightkins down there.
When you are helping out that soldier ghoul who wants to know if his friend is alive, use Stealth Boy instead of fighting those other Nightkins.
I only needed to kill two of them, I made a deal with their boss, snuck past them with a stealth boy to find the missing ghoul that Harland was looking for to get him to leave and then accessed his computer to get the info to make the Nightkin leave.