Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Janna is put of as being a weak champion. She is a support so she has minimal burst and limited surefire damage (as her main damaging ability is delayed skillshot), but she has aoe slow/heal as well as a shield that increases damage which makes her a very nice champ to have on a team. This build focuses on AP, but has health, damage and attack speed. In this guide I will talk about early game, mid game, and late game (as many guides do). This is my first guide, so criticism and comments would be nice and please rate (even if its down). Thank you.
This build is not meant for mid, but if you insist or are forced into mid by your team, get Howling Gale before Zephyr. The items remain the same, but your summoner spells might want to switch to heal and clairvoyance. When out of the starting gate, I like to get the health crystal first (to start your rod of ages). It makes her virtually invulnerable in an early surprise bush fight and gives her some pretty decent staying power. The Zephyr you got should help slow an enemy (in aforementioned bush fight) and you should get an assist if your lane mate is competent. Obviously this only works if a fight happens. Once you get Howling Gale, you are officially the harassment king/queen. Using it when you know you can hit a wave or a champ makes things really easy for your lane mate. Often times resulting in a kill or assist if done accurately and frequently. This should push your lane and harass enough to kill or force a blue pill. You should be one of the first towers down. After that, you should stay and defend your turret leaving your lanemate to help mid or the other lane. This will allow you to creep farm and level effectively, as well as defend a turret. Once you feel you can help, a tower needs help, or all of their first towers are destroyed leave and help team mates. At this point you should have Rod of Ages, Boots of Swiftness, and well on your way into your Archangel's Staff and you enter mid game.
Early Game Tips
Clairvoyance allows you to line up shots on enemies that are blue pilling. Use it often, its cooldown is pretty quick.
Use your slow sparingly. Only when you know you or your lanemate can kill them should it be used.
Use your shield. It helps attack damage. Even if no one is being attacked the shield can be the difference between a kill or a close call. If someone is harassing your tower put shield on your tower. It will keep it alive and give it a nice little attack bonus. Shield is also a nice tower falling spell. If you are attacking their turret, use it to get a couple more blows in and get more damage out of each blow.
At this point you should have average health, a nearly endless pool of mana, and some pretty decent AP, now is the time to shine. Your bread and butter spell shifts a bit. As you should be with at least 2 champs at all times, using your slow is essentially certain death for any stragglers you come across. With shields your enemies should go down quick, and your team is indestructable (almost). As more turrets start falling you should be getting close to some inhibitors (or they to yours). Just keep supporting and blowing creeps away, you should be an asset to your team. Use Monsoon as you see fit, if you have an ally that is low health, but still has good mana, use it as a heal. Its always best to have a nicely healed champ with you even if you sacrifice your ult. You and your group of champs are pretty much ungankable due to your shield, slow, airborn, and ult. You are golden at keeping your team alive. Pick up assists if you can. Your record should be around 0-3/0-3/10+. Assuming your team is good. Janna is only as good as her team. At this point you should have all of your items except Warmogs, or on the verge of getting Guinsoo's.
Mid Game Tips
Use clairvoyance to prevent your group from ganks or find gankable champs. Just because you are ungankable doesn't mean it isn't better just to prevent it before it happens. (Note: Clairvoyance CANNOT see stealthed units, but it can see in bushes)
Tell your team when fortify is up and have them call for fort when they need it (this goes for the whole game, tell them early).
You are a minion killing machine. Don't be afraid to go set a gale next to a particularly intimidating bunch of creeps (shield your tower first though, might as well prevent the damage). Just because you are an assist doesn't mean you can't just blast away if they try to gank you. (You are a speed DEMON at this point)
You're fast, you have immense AP, Health, and Mana. If you are winning guinsoo's will help you take down turrets/inhibitors quickly and even help with assists and kills. If you are losing, Guinsoo's helps you harrass and kill efficiently. Your AP should be through the roof with your Rod of Ages, Archangel's Staff combo. You are extremely potent, and deadly. Never let you're enemies underestimate Janna ever again.
Thank you for reading this guide. Please rate (to promote it or warn people of its suckage). Comment for criticism (as long as it isn't harsh and has something beneficial to say). Once again thanks! I hope you enjoyed this build.