Greater Glyph of Magic Resist We are going a bit tank with this, so having magic resist will make you more well rounded.

Greater Mark of Magic Penetration Because if you're building AP why not have some extra magic pen to hit harder.

Greater Seal of Armor Again....tanky...more armor.

Rylai's Crystal Scepter is a must to rush early and gives you 80 AP, 500 extra HP, and most of all now all you skills will make them slow.

Sorcerer's Shoes I try to get these after Rylai's because you can stack on the magic pen and it's always good to have movement speed when your Elastic Slingshot is on cooldown or it gets interrupted.

Hextech Gunblade Gives you a bit of AD and AP along with lifesteal and spell vamp to help keep you up in those crowded team fights.

Randuin's Omen Tons of health and slows enemies attack speed when they hit you, andd armor makes you more able to out DPS those pesky ADC's that have fast attack speeds. Also, you can't go wrong with an AoE slow to help your team smash running enemies in team fights and keep them from getting to safety.

Liandry's Torment Great for burning down baron and the AP casters that try to flash away and think they got out safe. Also, more health which is always a plus and more AP and magic pen. (You will begin to drop tons of enemies after this stage of the build.)

Rabadon's Deathcap And finally Rabadon's an AP builds most prized possession. This just helps you to trow dirt in their faces boosting you to a total of 448 AP!

I don't recommend Spirit Visage because you really don't need it when you have lifesteal and your bloblets along with spell vamp. Plus, at high levels 's bloblets heal him pretty good and you smash people hard with this build that you can kill off people then pick up the pieces to be ready to charge in again in no time.