I don't like filler, so I've compiled everything you'll need to win: runes, masteries, and moves.
You've got the armour, magic resistance and HP to help survive in a fight while using your R, the nashors is a twist, it works well killing minions in lane and champs by taking your AP (Ability Power) and using for basic attacks. I chose this because after E and R you have nothing. Aleast now you can attack with some damage and attack speed while it's giving more AP.

Take a point in each move early, poke with E and level it first you wanna be able to poke hard, it's one of Nunu's best abilities. Every item helps here with everything having AP and/or cooldown.

Max Q last, but again take a point as early as level 2. Normally I'd max this last but with you able to heal all the time thanks to the cooldown boots and Nashors, you can keep healing and keep poking, untill you're ready for a kill.

Max W second but take a point at level 3 because the boost will do well on a partner in lane. if you can help them they'll help you. With the slow from E, and the speed boost from this you should catch almost anything.

Always for every champ I've every played (most of them) max R at level 6, 11, 16. This move will be throwing out massive damage. If you start taking to much damage in a team fight, break the channeling with hourglass, or if your team is tanky enough, start with it, so people can't attack then use R straight out, I'd only suggest using this when the team is level 18. Take advantage of bushes. Like Rengar hiding ready to pounce, no one will see you using your R in a bush, they'll be confused as to why they're moving slowly. If you've using this on a single target, while hiding, after R hits, boost on yourself chase hit with E, they should be dead or close to it.

I haven't used this build a lot, but 2/3 matches I did use it, I come out at 20+ kills -5 deaths, +20 assists.

I also find if teammates are champs like, Blitz, Thresh, Trist ie anyone who can force an opponent to move, it helps you'll need to keep them in your radius for R.

Amumu, Katarina, Gangplank, all work well too. With the high dmagae, large area, and some good timing, you'll kill a team of 5 in 3 seconds.

Sorry there's not more filler I hate it, if you have questions message me on facebook.
https://www.facebook.com/Phenic.Raider