Hey dude, for those who don't know Nunu rly good a longer description about items, Summoner Spells etc. would help. Your choice of items isn't bad but u always go for fast catalyst so I don't understand why u take clarity as your summoner spell. Better take flash as 2nd spell. In combination with your ultimate (Flash into enemies --> Ultimate) it's very easy to deal high damage to all of them.
Taking heal or ghost isn't bad but I would recommend exhaust because this slow very much. Exhaust --> Ultimate ensures a minimum of one kill.
Improving these points will make your guide getting a lot more attention and votes.
Write back if you improved these points.

Grtz,
Just a Reflex