I take:

9 Greater Mark of Insight for magic penetration.
9 Greater Seal of Replenishment for mana regen but you can take AP.
9 Greater Glyph of Force for more AP : Why i dont take more CD ? I have 39% CD without...
3 Greater Quintessence of Potency for more AP again.

I think 9/0/21 is the best options. She's magus!

Items you should have:

Sorcerer's Shoes you can take that if you want more Magic Pentration and you have only 24% CD
Mejai if you are fed.
Abyssal Scepter because you haven't Sorcerer's Shoes.
Void Staff can be use.
Lich Bane he's good with Lux's passive.
And other AP items

You can switch LIGHT BINDING and LUCENT SINGULARITY if u can't do LIGHT BINDING because enemies stay behind mignions.

Spells you should have:

Ghost You can Chase and run.
Clarity You need mana and you mates too.
Ignite This spell can help for kill at the beginning.
Claivorence For 100% chance to kill with ur ultimate.
Flash but I prefer ghost.
Teleportation can be practical

This is my first guide so let me share your reviews and I apologize for spelling I'm french. :)

Fors4k3n.