Nunu is an amzingly good hero. You don't need to get a lot of kills, but he wins games.
Here's my strat, won 8 games in a row with it. lvl 14 summoner.
Teleport / Smite
First item: meki pendant.
In the beginning, tech consume and stand near some creep spawns. You can smite/consume early, gain some easy gold, and have consume cooled down for the first wave of creeps.
Consume + pendant allows you to stay in lane no problem, smite and consume creeps and you farm terrifically.
You only need one level of consume, maybe two if they're harassing you badly.
After that, two options:
If you have an Ashe or a Jax that is doing well, blood boiling them all game wins the game.
Nunu is a one trick pony, his ult OWNS.
If you can pull it off!
So, if you're playing a chogath or something, it can be tricky.
But, just keep stacking ability points. Make a philosopher's stone, then:
mejai soulstealer (really helps - you'll get lots of assists with an AOE ult anyway)
abyssal sceptre, etc
you don't need health regen due to consume, you don't need mana with a philosopher's stone (free gold helps too), nunu is pretty fat so he doesn't need much to tank (rylai does it), and if you can pull of your ult in team fights, you can take out 3/4 of weak heroes health or 1/2 a tank. Not only is that absurd damage, but when you start up your ult, the only thing people can do is flee or try to stun you (continuing to fight your team is suicide), so you've essentially got an AOE stun for free. That, and you blood boil your MVP = gg.
If you're fighting a debilitating amount of silence or stun, this strat won't really work. But, you can successfully avoid most stuns and pick off people in the middle of a team fight - you don't have to start with the ult.
This will result in lots of scores along the lines of 8-5-20.
I recommend defensive and utility masteries. You don't need attack for Nunu you just need the ult and a team.