Janna is a peel heavy mage support who, thanks to her ability to shield people and negate engages can be difficult to lane against and even harder to optimally engage against her team, especially since she has the ability to reset an entire fight when played well, while simultaneously displacing the enemy team.
Passive – Tailwind
Tailwind: Increases the Movement Speed of all allied champions moving towards Janna by 8%.
Q – Howling Gale
Summons a whirlwind, which deals magic damage to enemies in its path and knocks them into the air for 0.5 seconds.The whirlwind can be charged for up to 3 seconds. For each second it charges, it deals bonus damage, knocks up for an additional 0.25 seconds, and travels 35% further.Activate again to release the whirlwind early.
W – Zephyr
Janna summons an air elemental that passively increases her Movement Speed and enables her to pass through units. She may also activate this ability to deal damage and slow an enemy's Movement Speed. The passive is lost while this ability is on cooldown
E – Eye of the Storm
Shields an allied Champion or turret for 5 seconds. The shield absorbs damage and grants Attack Damage until it breaks.
R – Monsoon
Janna surrounds herself in a magical storm, throwing enemies back. After the storm has settled, soothing winds heal nearby allies while the ability is active.
In lane Janna is far less powerful without her shield, and as a result so is her partner. Most AD Carries or supports can use the cooldown on the shield to harass their opponents while she doesn't have access to it.
Janna dies quickly, and provides a lot. If you see the opportunity to CC and kill her in a fight, it's usually a good idea to take it, before she can use her Monsoon to reset it for her team, however, do not do this if if you have a better pick on a higher priority target (I.E. the 11/2 Vayne)
Janna loses her movement speed bonus if she uses the active of Zephyr, baiting it out can make picking her off far easier than it would have been otherwise.
If you catch Janna using her Howling Gale in lane before six, as a duo with kill pressure, use this time to get lane dominance. She can't peel anyone off of anything without it or her ultimate and becomes a shield bot until it comes back up
He just does a lot of damage from a safe enough distance to not care about anything. If he stuns either Janna or her ADC, they die, before six, and depending on how his ADC plays, even after six. He doesn't need to get on top of Janna to kill her and she doesn't like that. Once he learns Pyroclasm, there's nothing a Janna lane can do about him
Black Shield basically. She can prevent any sort of peeling or interrupting with her Black Shield, and if she lands her Dark Binding then there's just no way that a Janna lane wins the trade. She can also easily use her ultimate on Janna and her partner, since she can black shield herself to prevent any sort of CC
Janna's Q won't save her rom Blitz's Q, and Janna is too squishy to survive a grab. If Blitzcrank lands one she dies, and if he chooses to not throw one for a long time, she has to play respectfully and reasonably defensivley, preventing her from poking the Blitz lane too heavily.
He can Q away her shield and the extra AD from it, and has a movement speed steroid that allows him to close gaps against Janna and her AD Carry. He can also interrupt dashes with his Pillar of Ice. His passive, Kings Tribute also prevents Janna from poking him out of lane
Being a support not named Brand, she doesn't do a lot of damage going into the mid and late game, and her early game damage can be mitigated by specific runepages, and whatever defensive abilities you have on your side. As the game progesses, chances are high you'll want a Banshee's Veil to avoid a knockup or slow, and/or a Randuins Omen to slow her. Frost Queens Claim is a good item to do this as well. She'll want to fight in chokes and narrow avenues, so you'll want to get pink wards and sweeping lenses to ensure that any bush ganks are seen.