This build is primarily for support, but is able to dish out damage if needed, as well as chase down fleeing enemies. If you prefer, you may switch out Rod of Ages or Rylai's Crystal Scepter for Zhonya's Hourglass or Will of the Ancients. Will of the Ancients will still give you 80 AP, but will also give you and neraby allies 20% spell vamp and allow them to heal themselves while fighting. If you want a little extra AP or armor, get Zhonya's Hourglass. Rod of Ages is very useful due to its HP, MP, and AP boost, although you must wait 10 minutes to fully upgrade it. And for the obvious reason, Rylai's Crystal Scepter will slow down enemies trying to run, which will easily allow Janna to catch up and finish them off with Howling Gale. Since the masteries give 10% cooldown reduction, the items only need to stack to 30%, which makes Ionian Boots of Lucidity and Athene's Unholy Grail perfect for Janna to get her ultimate out faster and stronger.

My reasoning for Promote is to help hold back enemy minions and champions, as well as take down turrets faster. The promoted minions also make an excellent distraction if you need to chase an enemy past the turret. Like many mages, Janna can use her mana quickly, and Clarity will allow you, and nearby allies, to keep fighting back. If you wish, you may replace either spell with Heal to be more supportive whilst your ultimate is on cooldown.

Thanks for reading this! Please tell me what you think, and anything I could do to improve since this is my first guide. I hope you find this useful.