I take almost with every champ Ghost and Exhaust: its good for killing, running and chasing. In Nunu & Willump's case, it would be possible to change ghost with clarity, but thats quite useless in late game.

you might wonder why i didnt put any points in defense, but this is compensated by runes and item build. cooldown reduction and magic pen is very handy.

magic pen for marks, then armor seals and magic resist glyphs: its cheap and good.

Allways open with Ice Blast. it deals lots of damage and slows like crazy. follow up with Consume and blood boil, thats the best. max out ice blast first, its your only damage skill besides your devistating ulti. just evenly max out consume and blood boil, its what u like the best.

Change dorans ring for rabandons deathcap in late game. With this items you'll have lots of health, good amount of magic resist, armor, and a very nice movement speed combined with Blood Boil. Ability power is also very high for compared with the amount of health and armor/magic resist you have. With Rylai's Crystal Scepter you will completely devestate the enemies movement speed, so thats very good for chasing or running. With Boots of Swiftness, Force of Nature and blood boil you will always be able to keep up your enemies and pull them into a fight, because they cant run because of ice blast.

Cast ur ulti while u stand in the bushes: enemies will realise it to late and you will have a certain hit. combine ice blast, exhaust and ulti for maximum damage, for a sure kill. but be careful: besides your ulti your only damage is done by ice blast. when your ulti isnt ready, it isn't easy to kill 1 on 1. so, when u try to force something or gank, do it in team. Nunu will slow the enemy chapion down with ice blast, boost his teammates with bloodboil and when he's at low health in a battle, quickly eat a minion and fight on!