I'm a big nunu fan. And i see him as one of the only Mid laner that can be opposed to Heimerdinger without problems. But i strongly disapprove of so many AP items. You dont need that much to deal butload of damage. And you dont need to be in a bush to use your ultimate. I'd suggest going Rod of age, then frozen hearth (your CDR would be capped, making your ult almost spamable), then i'd go banshee(The first stun you get get dispelled). You should never need more than that to be good with nunu. After i'd go Abyssal/lichbane/zhonya for magic resist, AP and the active of zonya.

Just my two cents.