League of Legends (LoL) Question: Doesen't Janna benefit anything at all with defensive stats?

you can either play defense or Ap

but i prefer Ap (full Ap) why? because i tried using her 2nd skill (zypher) it damages ranging to 280 plus your current ap and when clash use your 1st Howling gale then be patient on 4th monsoon you would rather kill a combo of triple kill because of the magic damage of your monsoon

Utility/CDR items are pretty standard. Mikaels crucible into locket with frost queen. Some go for small amounts of ap too. Full AP I wouldn't recommend tho.