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.
RUNES: You need AP rune just to have 200 AP at late game. Magic penetration to maximize your damage and some mana regen because this pg eats too much mana.
SKILL: Just a point on bandage toss, then maximize Curse, Tantrum and Despair. Bandage is the last one.
SPELLS: Telport is one of my favourite spell, because it allows you to come back base and buy items without losing exp. You'll be overequipped then enemies champs on early game, so it will be easier to survive or kill em.
Flash is incredible, you can go out of range/aoe skill ( like nunu's ultimate ) or telport yourself near them and kill with your aoe dmg, while walking.
Doran's shield + Health pot.
Chain vest / Megatron cloak : Depends on enemies.
Sunfire cape / Force of nature
Sell doran's shield -> start Guardian angel
But, imho, you cant follow an "item purchase order guide" because it depends on a lot of thing. (if you make 5 kill in the first 10 minutes of play, youll stack more dmg ( sunfire and scepter) before stacking guardian angel.
If you die a lot of time you'd better take some def more.
AP : 43 (Runes) + 70 (Abyssal) + 80 (Rilay) + 3 (Archmage masteries) + 4% ap (Ardor masteries) = 204 AP, it means you deal 2 % more dmg with your W skill.
Magic penetration is high enough to do good damage, tnks to boots, runes and abyssal scepter.
Your survavibily is pretty good. You will have 200 armor and 200 magic resist.