Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This guide will explain to you and show you what is necessary to use an AP NuNu build. While it may seem rather pointless, it really isn't. With NuNu's spittle attack and his ultimate, it deals enough damage to enemy champions, especially in team fights and in 1v1 scenarios, plus, while using Blood Boil, your attack speed more than makes up for the lack of damage that you'd normally deal to turrets.
Early game break-down: You may be looking at this build and going; "WTF, is this guy ******ed? He built NuNu AP, and then went defensive mastery and rune builds." It's true, my friend told me I was a little bit loopy for doing this, but I have good reason. Building NuNu's masteries and rune-tree for a tank character gives him high survivability while being able to shell out the damage.
In the early phases of the game, it's important to use the icy spittle to your advantage as much as possible, leveling it up as soon as it becomes available each time, and capitalizing on the extra damage that your Mejai's will eventually add, as well as the magic penetration that the Sorcerer's Shoes will give you. Consume will be used to mainly heal yourself without having to leave your lane, and Blood Boil can be used to chase enemies.
Obviously, NuNu's ultimate is to be used cautiously, as it can be canceled by some forms of CC. With the new champion, Garen, I've witnessed being silenced in the middle of it, which doesn't cancel the ability all together, but ends it extremely early, gimping the damage. Obviously, team-fights are the places to use this ability, and it will absolutely decimate entire teams if you can get it off before being CC'd or killed.
End-Game: By now, you're receiving the full benefits of the armor and health runes that you put into your book. You will be shrugging off a decent amount of damage, and will be able to stand through most of what the enemy team can throw at you. Literally, you're probably going to be playing the same way as early game, using the Icy Spittle for slowing effects, taking advantage of the reduced cooldown, and then going in for the kills once the teams get into massive fights. Your worst enemy by this point is probably stuns and the Summoner Ability Cleanse, seeing as how it will allow people to escape outside of your slows and ultimate ability.