Pros
Superb Harasser
Awesome Damgage
Great at any lane
Can stay in lanes longer than most.
Great counter-jungler.
Cons
Might have bad timing with ultimate.
Semi-weak beginning.
Weak mana regen early on.

Using these runes allows you to deal good damage early game. With combined items you'll scare your opponents.

Runes you could switch out:

Go all Potency runes.

Add mana or mana regen.

Or grab more Magic Pen.

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Recommended

Flash
Having this spell is a must for all summoner's but some find it difficult to use. If you are the one that finds it difficult to use grab Ghost instead.


Ignite
Ignite is a very painful DoT. It deals True Damage over Time, damage that can't be negated by Magic Resist or Armor. Also if traited in Summoner's Wrath it gives you a nice +5 to AD and AP.


Ghost
Finding it difficult to use Flash grab this instead. This will speed you up and get into any lane quickly, or perhaps grab that escapee and get the kill. Traited in Summoner's Wrath this gives you a 8% bonus speed. (For those who don't know math very well Ghost is 27% without Summoner's Wrath adding in Summoner's Wrath makes it 35%. 35% - 27% =8%)

Okay to use

Clarity
This spell is okay to use. Mainly in early game where you find your self without mana but later in-game it becomes useless.


Heal
I would only see a noob use this perhaps your not a summoner level 30 yet or maybe not 20 or even so you probably don't know that he can eat minions to gain health.


Revive
I could see someone use this if they went with the off-tank mastery build. Generally don't use this unless your a tank or newb.


Promote
Nunu bot uses this in the intermediate and I can see why makes him and siege minion a terrible duo of power. Use this if you feel confident that you don't want Ignite.


Smite
Typically used in the jungle. Smite can be used if you want to go get blue buff early on. Or counter jungle someone else.


Surge
I could see someone using this instead of Ignite. Grants AP and AS varying on your level.


Teleport
Generally I'd only recommend this if you went with Revive instead. STAY AWAY if you didn't grab Revive.

Don't use

Cleanse
If your getting hit by a lot of stuns, slows, silences, and/or fears grab a tenacity item like Mercury's Treads or generally good for this build Moonflair Spellblade. These items reduce these effects by 35%.


Clairvoyance
If your worried about someone ganking you grab a ward or two.


Garrison
If your in Dominion, don't use this. It's really good for support based characters but not for Nunu.

Build 1


This build primarily focuses on grabbing CDR (Cooldown Reduction) items and maxing it out. This will give your Ice Blast a 3s CD when maxed out. Also grabbing health, AP and Malady greatly increases your harassing mid-end game.

Why grab Malady?
Malady increases your AS (Attack Speed) and applies a DoT (Damage over Time) and cause a Magic Resist debuff on your opponent meaning that when you hit with Ice Blast it will do even more damage.

What to switch out?
With Build 1 you can switch out items from Build 2 or switch out Malady with any other AP items. My faves are Rabadon's Deathcap can't have too many of those, Zhonya's Hourglass or Abyssal Mask to replace the Magic Resist debuff of Malady.

Build 2


Like Build 1, this build also focuses on getting CDR items. But more AP, and less Magic Resist Debuff.

Why Nashor's Tooth?
The CDR from it is the highest in-game, combined with AP and AS, it's good item for anyone who needs a bit of umph in their game.

Why Mejai's Soulstealer?
Even though at max stacks you get a CDR, it's stacking AP. It also if you get enough kills is the highest AP item in game. Giving you +20 AP on the spot and adding +8 for each assist or double for each kill.

What to replace in this build?
This build can have items replaced in from Build 1. If you grab Mejai's Soulstealer first don't worry about Nashor's Tooth, only if your getting enough kills. If you grab Nashor's Tooth first you can sell it later when Mejai's Soulstealer reaches max stacks. Fav items to be replaced Trinity Force really great item for this build or replace with fav items from Build 1.