Zatanna is a champion that can be used either as a support or a power nuker.
maxing out Q and E while keeping W at the lowest level (while not putting points into the passive) will help be a very powerful nuker. W is very useful in getting away, helping a carry get away, or to chase down your enemy.
Always use your ulti to get rid of the enemies biggest dmg dealer so your team can kill them in a matter of seconds. With the cooldown items, the ulti can be used whenever, even if it's just to get away from an enemy.
If you are being picked off way to much by assassins or other attack damage users, suit of sorrows helps alot to survive as it adds 100 attack armor.
Ra's al Ghul's robe is another good way to survive.
If your team needs more support than damage output, then some of the optional items provide auras.