Eye of the Storm can be used on allied turrets.
Quickly firing a Howling Gale without the charge up can be used to heavily disable the other team.
Timing Janna's ultimate can push enemies away from a wounded ally or even separate enemies.
Save an interrupt ability for when Janna uses her ultimate.
Listen for the charge up sound on Howling Gale in case Janna is trying to hit you with it from off screen or from brush.
Janna's at her strongest when buffing another ally. If you can harass her ally, it will weaken her ability to fight you.