Here is another of my builds for you who would like a nunu that can be a great killer.

The secret to be a good killer with any mage or magetank is a rabadon's deathcap, since it gives you 155 ap and 30% of your total ap. As u can se i didn't only take a full ap build nor a full tank build though i'm pretty close to be both. So i guess is kind of a hybrid build that is perfect for both causes. Nunu is capable to be a good DPS as far as a tank, the zhonyas hourglass is good for when you get ganked and you have to get your team mates over to help you also for when you inisiate a team battle. as for the rod of ages it is good to buff up your health and mana whitch Nunu both really need. as i said the rabadon's is to get your AP as high as you can since you want to do maximum damage. The rylai's crystal shepter is also a good health item for any tank mage. the guardin angel is just a really good item for a tank that need magic resist and armor. The revive abillity is just a really good and awesome perk for your Nunu. That just concludes the nunu build, please give me feedback, i would really APPRECIATE it, thanks for viewing it have a good game.

GOOD TANK
GOOD PUSHER/LANER
AWESOME battle starter


BAD AUTOATTACKER

NUNU is a good jungler since he has consume whitch gives him some awesome damage on minions/creeps and also returns health. He also uses bloodboil to get a movement speed buff and attack speed buff.

My personel favorite is the ones i choose but you can also use flash instead of ghost or even teleport. IF you a jungler you might want smite but it's not a must since you have your consume.
Some people might want clarity but Nunu's passive gives him a spell that's free from mana every 6 strikes so i find it stupid to use.

THE runes i choose are all tank runes exept the glyphs whitch i choose AP since you basicly want to start out with some AP. Otherwise you will ned the tank runes since they really work better with nunu in both early and late game.