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.
HOW IT WORKS
Six Phantom Dancer always works on all champions that have the ability to carry. Having such on Veigar and Kayle works well because they both have enough CC to hold off the enemy while you work wonders with your 100% crit. Also, the movement speed will help you escape any ganks. For off-tanks and such, boots DO stack, so that is why I get six Boots of Mobility so I can ulti and then run extremely fast.
Thanks for reading, I hope this helps you.
Please try before voting, and I don't accept constructive criticism.