If you're going against an AP mid!
Get a Null Mantle for the AP mid(2pot) because AP mid can barely touch you and you already do enough damage by flat ap runes. After that get an Abyssal Scepter but, build the Negatron Cloak 1st. After that get a Wits End for sustaining in fights even better. Get a Rabadons Deathcap or a Rod of Ages. It depends if they can easily kill you so I rather get Rod of ages before Deathcap.
IM LAZY BUT ITS A VERY GOOD BUILD. SO USE IT BTW GET Sorcerer Shoeswith red enchant