League-of-legends – How should I build AP Janna

It depends on who is fed on the enemy team and how many characters they have in that role.

For tanks: If they have Yi jungle, Tryn top and Kog bot, I should go for Thornmail and Frozenheart.

When the AD carry is killed 5 times in 20 minutes, and their AP mid got free farm and/or kills, you need to focus on MR.

If you're an AD carry and the enemy has a Thornmail, get a Wits End. It counters it (or so I have heard, I dont play that much as AD carry).

You generally dont build a last whisper after the black cleaver because flat Armor Pen makes the % armor pen less effective.

Say they have 100 armor. Black cleaver removes 45 armor, then your masteries remove another 6, then your 10% armor pen from masteries plays in, leaving the person with 45 armor. 40% of 45 armor is only a 10% damage increase, if that. And thats coming into play if the person actually bought enough armor to put them at 100.

Generally another AD item will give you more speed or sustain while providing more than a 10% damage increase.