This is a very special version of Nunu, which is mighty without his ulti. On 18 lvl snowball deals 275 + 601 = 876 damage. Then Nunu actives Blood Boil so he becomes (on 18 lvl) extremly fast. Because of the large cooldown reduction (35 %) Nunu will cast the next snowball in 4 seconds. Remember that snoball slows the enemy for exactly 4 seconds, so there is no escape. In conclusion without ultimate spell Nunu can deal 1700 damage over 4 seconds + physical damage on Blood Boil and slows the enemie.

It's too low damage for you? We have also ultimate spell (at 18 lvl 1125 + 1500 = 2625). It is extremly high damage. The guide how to use this spell and very helpful Consume will be presented in paragraph skill sequence.

Pros of the masteries:
-> summoner spells are improoved
-> cooldowns are reduced
-> there is the increase of ability power and critical strike (while nunu attaks on Blood Boil)
-> very important is the increase by 4% magic damage
(LOOK, WHEN NUNU DEALS 900 DAMAGE WITH SNOWBALL - WITH THIS MASTERY HE WILL DEAL 936, AND IN THE CASE OF ULTIMATE SPELL THE INCREASE IS FROM 2625 TO 2730)

Again we have:
-> increase of ability power
-> cooldown reduction
-> increased magic penetration

1. IOANIAN BOOTS OF LUCIDITY
This build needs cooldown reduction (15 % from this item). It is a must, unchangable for this build.

2. CATALYST THE PROTECTOR
In early game Nunu really needs mana. Without this item he will go back to the city every 2 lvls. Then this item is needed to make Banshee's Vail

3. MEJAI'S SOULSTEALER
There is very important to collect 20 stacks. It will give our Nunu 160 AP and cooldown reduction. Without many stacks Nunu can be too weak.

4. BANSHEE'S VAIL
It's a must for Nunu. His ulty can be stopped with every snar, stun, fear, so blocking one negative spell in many cases make cast of ulti vailable. It also brings some HP and mana.

5, 6, 7: LAST THREE ITEMS FOCUS ON ABILITY POWER AND A LITTLE BIT ABOUT HP

It is a great solution, but what will happen if enemie team have too much Magic Resist?
Then you should change one of theese items (perhaps Rylai's Crystal Specter) to VOID STAFF.

Fact that all players should take as quick as possible the snowball skill is obvious. Increase of movement speed is important, but for me the nexr skill in queue is Consume. It really helps Nunu to stay on the line very long and sometimes it rescues Nunu from very dangerous situations. REMEMBER: With teleport enemies don;t have any break !!!

NOW ABOUT ULTIMATE:

1. Don't use ulti only on team fights. You can use it against one or two enemie targets if they don't have more than one snar/stun (REMEMBER: one negative spell will be blocked)
2. If you want to use ulti on team fight do it properly: hide and then push R ;-) or use it after your enemies use their stuns and snars on the end of the team fight