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.
to optimize this build you have to focus on being part of lots of hero kills and to farm the minions early game for AP. once you have your mejai always be prepared to run away from battles that look like they arent in your favor to preserve your stacks. once the build is complete and you have 20 stacks you should have over 900 AP doing almost pure damage do to your magic penetration and with a brilliance potion have 25% CD reduction.