For skills you want to max Consume first for the epic heal and then Blood Boil for more ASPD. Get one level of Ice Ball at 9 but don't get any more levels because it's too mana intensive. You only need one level of Ultimate because it does piss poor damage.
For items you want to focus on attack speed and lifesteal. Through my rigorous testing of Nunu builds I've found the best one to be Malady > Beserker Greaves > Malady > 2x dagger > Vamp Sceptre. Your enemies should be ending, but if it isn't then build one more malady before building a Wit's end and a Thornmail if they have no DPS.