Hybrid Nunu Melee, Support. Is focus on Support Nunu, and cast fast to slow oponent while attacking.
Nunu is not a Full mage and is difficult to kill a player even with a lot of AP, with attack speed can kill minions, buildings, and champions easy while suport the team; Bloid boil give a very well extra bonus.
Smite is optional, i think is good to farming at start the game combine with consume.
Maybe a Exhaust / Flash is the best option because with Flash can enter the battle easily to cast the ultimate and is a very well escape system. Nunu dosnt have a escape skill yeah slow but i think is not enough special 2 or more vs 1.
Usually i change the randuin omen for phantom dancer, Cleaver, brutalizer, last whisper, depends the game. The active skill randuin omen is very good, especial before cast the ult.
Maybe the build is better with other champion with more support to kill and damage, but is very funny use the Nunu mage is boring.