Nunu can hassle enemies and distract their farming effectively with Ice Blast. With the armour and MR+ runes, Nunu can initiate and take a few shots to lure the enemy ADC. But Ice Blast drains mana fast; I'd recommend not using it when close to the enemy turret. Beware if the enenmy champs have cleanse skill. Make them use it (i.e. throw some casual Ice Blasts to chip off some HP) before you initiate so you can slow them down when it's absolutely necessary.

My usual way of initiating a fight in the bot lane is to ice blast --> blood boil my ADC --> and ULT. When used in the right location, it can reduce the enenmy's health drastically or even get a kill.

While survival isn't a big issue for Nunu because he can get away with Ice Blast and Consume, don't throw yourself in when a big team fight ensues. The item build isn't tanky enough. Watch for your tanker to go in and ice blast and blood boil as necessary.

General supporter tip: if on the purple side, Baron can be warded from the purple jungle (behind the elder lizard). Make sure you ward the bushes in the jungle (namely the ones right next to the inner mid turrets) and the bush that leads the blue jungle to Baron.