Either pick him off at a distance or hit it with full close combet force.
What is full close combat force? Well, I would suggest a Terminator chaplain With 7 LIghtning claw Terminators, with furious charge. Let's say you knock the Nightbringer down to 4 wounds; you then hit it with 28 S5 I5 attacks that re-roll to hit and wound. That's 21 hits and about 6 wounds. That's not even enough to kill it. So, I would suggest loading the squad with 4 Lightning claws and 3 thunder hammers. The 4 LIghtning claws are to hit the nightbringer and cause about 2 wounds, and the thunder hammers can then safely finish it off - the Lightning claw termies should be removed when the Nightbringer attacks back. With the 9 thunder hammer attacks, which will cause 6 hits and 4 wounds, the nightbringer will take at least 1 wound, ensuring that next turn at least, it will be killed.
But that's pretty stupid. You're probably going to lose the crusader, at least 6 of the termies, and you'll probably lose the chaplain as well. That's well over 360 pts you'll lose.
When I play against the Nightbringer, it's usually with 2 Force Organization Charts, allowing me to take 2 relics or artifacts. I can take the book of salvation and cup of retribution, and put both of them in the area of attack. That's +2 attacks for everyone in range. So I'll take 8 Terminators in a Land Raider Crusader (furious charge, Lightning claws), a chaplain on a bike, and a LIbrarian who deep-strikes witha command squad near the assault. On the turn I plan to charge the nightbringer, I reveal both holy relics, and charge all 8 termies and the chaplian on bike into the god. The chaplain obviously can't do anything, but the 8 termies hit with 48 attacks. 36 attacks hit, 11 wound, very good chance of a dead nightbringer, as only 5 need to be failed saves.
That's pretty stupid too, as it poses so much risk, and if something goes wrong that's a lot meat for the nightbringer, but if it works, the whole area around the termies will be a death zone. In 3,000 pts, that's a lot of necrons for the termies to eat up witht he claws.
As for smaller games, try to phase the necrons out! In games of 1,500 pts, that's less than 1,200 pts of necrons to kill. In games of around 2,000 pts, you shold get devastators and other heavy weapons like lascannons and missile launchers to hurt the nightbringer before getting some powerfists in combat with it.
