Build and Runes
In lane, if she shields herself, it leaves her carry more open to harassment/being killed.
Interrupting Janna’s ultimate by using a stun/silence/knock-up/displacement will stop her from using her AOE heal.
Try to avoid Janna’s tornado by listening to the wind up and predicting the direction it will go.
Take care of your positioning when you think she may use her ultimate, as it can displace you to an unwanted place.