Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
My own variation of the two best builds here(the Harpsichord from Hell and Support/DMG Guide - Passive+Lichbane+High AP) , combining concepts of both to my preferences.
the tear of the goddess can be built sooner to get more mana and get the boost from archangel fully.
zhonya will be hard to get normally if the game doesnt last long enough or farming longer. Deathfire grasp its a good choice if the other team has those crazy tanks with millions of hps.
I dont like Mejai's in the build because it depends in getting stacks to be efficient, and if the things go wrong its a bit hard to get it to the sweet point of AP, i prefer a direct route to lichbane and archangel.