Janna excels at disengage — don't make predictable engages, she cancels them all with Q or R.

Her R makes her vulnerable because she channels: interrupt it with CC (or flank to force her to R without value).

Janna is very squishy and weak in direct combat: if you catch her alone, she dies easily.

Multiple engages (two CCs back to back) are effective: she can cancel the first but not the second.