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.
With my runes i make it simple, i get 9 magic penetration marks, 9 mana regen per level seals, and 9 cooldown glyphs. This makes it so by level 18 my mana is regaining quickly, so i can put out more ice blast's to rack off kills. For my Quint's i get 3 Flat ability power. Getting theese runes is esential to creating a massive Ability power damage nunu.
For my masteries i have a basic 9/0/21 taking improved exhaust in offense, and improved ghost in defensive, i take theese because i use the ghost to get close in a team fight if im too far away, and exhaust to slow down their tank, or their heaviest DPS allowing me to make sure my ult will trap them and deal massive damage.
Pros / Cons
The pros of this build:
Heavy hitting ult
Easy kills using ice blase
massive mana regen
The Cons of this build:
take's damage easily
doesnt have as much armor/magic resistance
in the beggining of the game i take Dorans ring, for the improved health and mana regen, right now i am not to concerned about having ability power, with this build it is a great idea to lane with a ranged or a tank. start by last hitting minoins and harrasing the opposing team's champions using Ice Blast. go back to base when you have around 1,500 gold and pick up your Tear of Goddes and your boots.
By mid game, you should have your Sorcere's Shoes and Archangels staff, this is where it get's fun, laning with shen, Ammumu or Rammus is a great idea for playing a mage nunu, This is because ammumus stun or Shen's and rammus's taunt is a great play for you to get in there and cast your ult. By the time you have Sorcere's shoes Archangels staff, and Abyssal sceptre your ultimate will be doing a massive amount of damage (i've had my ult at max doing, 850 plus 1350 damage) this is a great way to pull off multi kills.
Near end of the game your build should be complete and your ult will be something they'll be running from, it is a great idea to have exhaust and use your ice blast for the double slow to get in there and knock some heads together using your ult.
Thanks for listening, and trust me evertime you run in people will be shouting "Here we go" again.