General Item Build
Introduction to Janna's Kit
Janna is considered one of the most prominent supports in the League, more specifically the leading support in disengaging. Her passive, Tailwind, increases the movement speed of allied champions moving towards her by 8% in a radius around her. Empower her passive with the Captain enchantment for her boots for an additional 10% movement buff in a radius around her. Janna's Zephyr ability passively increases her movement speed, so she's usually the fastest champion on the team, giving her the opportunity to constantly implement her passive.
Tailwind might be most effective as a disengage, especially when coordinated with her area-of-effect, knock-up ability, Howling Gale, but can be used offensively to help your team engage. Using Zephyr as an active ability allows Janna to engage an opponent and dramatically slow them for three seconds. Although Janna will lose her movement speed buff by expending her Zephyr, her approaching teammates will still benefit from Tailwind, allowing them to pick off the crippled enemy. Since Janna's movement speed will diminish significantly after casting Zephyr on an enemy unit, it can greatly decrease Janna's survivability if she needs to suddenly disengage. Zephyr is extremely effective in the laning phase, but use of this ability in a team fight should be considered methodically, and complemented by positioning. Zephyr is only effective in a disengage when you, your marksman or your team as a whole are disengaging from one, sometimes two enemy champions. Instead of using the Zephyr active ability to disengage from the enemy team, try using Howling Gale to knock up the most mobile champions to discourage them from chasing.
Unlike the Zephyr, Howling Gales only consequence is a lengthier cooldown. Howling Gale is a 0.5-0.75 second knock-up ability - lighter control over the target, far less debilitating than her Zephyr, but Howling Gale is area-of-affect and is most effective when interrupting multiple units. While this might seem pointless in comparison to Lux's or Morgana's lengthy binds, Janna's passive ability in coordination with this area-of-effect disruption allows for far more successful disengages. A well-placed Howling Gale can serve as additional crowd control during engages, as well as immediate interruption of static ultimate abilities (Miss Fortune, Caitlyn, Malzhazar) and can interrupt assassins/tanks who jump on your weaker units (Rengar, Diana, Kha'Zix, Sejuani, Leona), although it takes a considerable amount of practice to react and instantly release a Howling Gale on time.
To further protect an allied champion, Janna can cast Eye of the Storm which will absorb damage that might otherwise kill them. For example, many mid-lane champions rely on the summoner spell Ignite to finish off their enemy after a burst of damage. Janna's Eye of the Storm can completely negate the effects of ignite, and can help a teammate survive in a scenario where they normally wouldn't. Eye of the Storm is also an attack damage buff. Janna can shield the marksman when attacking enemy champions to deal extra damage with immunity to their initial counter. Early game, her shield can be used more defensively, but later game you will find the shield used mostly offensively. It is best used by a champion with a generous amount of attack speed who is also building armor penetration. Nearly every marksman works well with Janna, but Janna should not be limited to just shielding the attack damage carry. Shielding Master Yi during an all-in team fight can have a terrifying impact.
As a last resort, Janna's ultimate ability functions as an area-of-affect push and heal. The perfect defensive ability to instantly counter an assassination on your marksman: instantly pushing the assassin away while giving your marksman a bit of a heal as they disengage. This ultimate is extremely dynamic: use it to push enemies into a turret or your oncoming team, to heal your allies in battle, to separate the enemy team or use when diving a turret to help survive turret damage. Janna's ultimate (similar to Bard's ultimate) can actually assist the enemy team, so be careful not to push an enemy unit to safety.
Janna's kit is completely balanced between offensive and defensive; she can take on both a passive or aggressive play-style depending on her opponents or how her teammates are playing. One of the most important aspects to keep in mind with Janna's kit is that she cannot deal significant damage without the aid of a teammate, her carry or an attack damage unit specifically. Much of Janna's worth comes from making her carry invincible while improving their damage output, ubiquitous protection and buffing of her teammates all while crippling the enemy.