This is my very first guide ever. Please leave feedback so that I may make this guide better. I am open to any suggestions. Please no trolls.

Nunu is a very strong AP Caster. His abilities allow him to deal significant damage (ice blast) and also heavy crowd control (super slows).

In dominion he has the ability to completely dominate his opponents. I try to maximize my damage output by concentrating on magic penetration and ability power but also trying to make sure I can sustain myself during team fights (durability). I will explain how I do this in later sections in this guide (runes, masteries, items, and skill sequence).

Pros

Strong Mage
Potentially high durability
Can be played as support
Heavy CC
Great AP Ratios

Cons

Low base movement speed
Ult can be cancelled very easily

RUNES
For my runes I take Magic Penetration Marks (9), +0.76% ATK SPD Seals (9), +0.99 AP/lvl Glyphs (9), and +4.95 AP Quints (3). I choose these runes for my Dominion Nunu Build because it gives me 21 bonus AP at the very start of the game which in turn will make my E (Ice Blast) do more damage at lvl 1. I also receive a 6.9% ATK SPD increase which helps proc my passive more often (free spell after every 7th auto-attack).

MASTERIES
For my masteries I go 22/8/0 taking everything having to do with magic penetration and AP in the offensive tree and taking armor and magic resit in the defensive tree. This ensures I maximize my damage and create a small base for my sustainability.

For summoner spells I take ghost for improved movement speed (essential in Dom imo)and ignite to ensure consistent damage output as well as to ensure kills.

I take my first three skills (Q,W, and E) right off the bat. I take the next two points in E (Ice Blast)to make sure I keep my damage high throughout the game. At level 6 I take a point in my R (Absolute Zero)which is his ultimate. I take a point in my R whenever possible (Levels 6,11,16)for obvious reasons.

Important Note:

There is one thing that easily distinguishes good Nunu players from ones that need work. That is keeping Blood Boil on a DPS carry or any friendlies in your vicinity when its off CD.

Farming is not as important in Dominion as it is in Classic but it doesn't hurt to farm some seeing as how you get more gold killing minions in Dominion than in Classic.

Use Consume to farm. It heals you and does massive damage to minions, like a built in Smite so to speak.

In Dominion as Nunu, you want to start at top and stay at top to defend. Making sure to roam around whenever possible (when top isn't being attacked).

Initiating:

When you're alone:
1) Blood Boil yourself or a minion.
2) Ice Blast your enemy (watch as their health bar drops significantly).
3) Auto-attack enemy until Ice Blast comes off CD.
4) Repeat...If low on Health, Consume a minion.

When you're with teammates:
1) Blood Boil DPS ally (or any ally of your choosing)
2) Ice Blast squishiest ally and begin chasing
3) Repeat steps until all enemies are dead, retreat if losing seems imminent.

MOST IMPORTANTLY:

Blood Boil should always be on CD!!!! Its too good for it not to be ;). With my runes Blood Boil is more effective and with Nunu's Passive (Visionary) you can ensure that you won't be paying much mana to be using Blood Boil so often.

BE AGGRESSIVE! Your damage output with this setup is very high and you are also durable. So don't be afraid to get in your enemies face.

One more thing!ULT IN A BUSH! I managed to get a Quadra Kill using this method. The enemy cannot see you or your ability until they are in the bush with you. Causing them to have to react even faster than if you were to ult in the open. I will post a video on this guide once I get LoL Recorder working right.