This is a quick little run over of how I play Nunu. I only use this in 3v3, so I'm not sure how well it works in 5v5. The main thing to remember about this build is that you NEED to get your Mejai's stacks. That's what will really help you out. The Ice Blast and the Absolute Zero do a ton of damage when you've got enough AP, and the AP also helps your Consume heal you. Mainly, this is just something I wipped up one day when I was bored, and it's marvelously effective. I really like AP Nunu, but he has much less survivability than having some tank items will give you. So I kept trying to figure out AP items that also help you tank. Now, Spirit Visage and Zhonya's Hourglass are two items like that, and you may want to grab them instead of some of the other items in the build, but the Spell Vampirism is what really keeps you alive. The major thing to remember with this build is that you always want to fight near an enemy minion or a neutral monster so you can get that awesome heal amount from consume. Nunu's Consume deals True Damage, however it still applies the Spell Vampirism. With Will of the Ancients, even at zero Mejai's stacks you will still heal yourself for over 1100 hp with your consume. Your snowball heals about 210, and your ultimate will heal about 650. Combined with the Heal summoner spell you will almost never have to leave your lane unless you're buying something.

With Max stacks of Mejai's Soulstealer your AP will be exactly 750.048, so 750. Your Ice Blast will deal 1025 damage, and heal you for 256.25. Consume will heal you for 1400. Absolute Zero will deal 3000 (exactly, weird huh?) and heal you for 750. This means that you will deal a LOT of magic damage and heal quite a bit. I was fighting an enemy Nunu one game, he Ulted and I triggered mine right after. I survived his, got most of the damage healed from mine, then I used Ice Blast and finished him off, as well as healing most of the leftover damage. It's a fun little build mainly because it can deal damage while keeping you alive and healthy.