What do do as AP mid vs full-AD enemies?
Say I'm Orianna or Syndra against a team with no major source of Magic Damage. Do I still rush Athene's? Or do I just get Chalice and then complete another item? Or something else altogether?
halp pls
Say I'm Orianna or Syndra against a team with no major source of Magic Damage. Do I still rush Athene's? Or do I just get Chalice and then complete another item? Or something else altogether?
halp pls
is a good alternative to 
Morello, or double dorans into zhonyas.
My typical build order as a mage against AD teams is something like Tear of the Goddess -> Zhonya's Hourglass -> Archangels Staff -> Void Staff. This build gives a nice balance of mana, CDR, survivability (shield, armor, stasis), and damage.
To compensate for the lost CDR, I use the following runes: AP Quints Armor Seals Scaling CDR Glyphs Hybrid Pen Marks
in place of one item of your build on each of your team. This cripples the enemy AD team. Since most ads rely on AA. Its also the reason you never want to be on a Full AD team.
rush seekers if they seem like they are going to be a problem but in lobby you should have armor yellows.
+ 
I usually get athenes if I'm against a AP caster too, the additional MR gets useless if you're laning against a ad caster in this case it's better to rush a tear or a morellos, both help you alot!
If I am on a high burst (at low level) champ, or if the enemy ad mid is not something horrifying to face (yas/zed/admorg/etc), or in otherwords, you can avoid most of their damage most of the time with no effort at all, then
.
Most of the time tho, I like
first, but if I can't really avoid poke or I am doing bad I will grab
before cap. If you can hold out a losing battle and stay in lane longer, you gain exp/gold advatage that can turn around the fight later.
Definitely core against all ad enemy team:
and another armor item if you require getting close enough for them to hit you back.
YOu get morello's instead. A better item cost efficiency speaking.

The game is balanced so the armor only block half of the damage. That's the spot where the armor is somewhat valuable and worth buying in reasonnable quantity for everyone.
If instead 100% of your damage is AD then, armor is just plain OP and you should not even bother about having a build. Just buy the op armors...