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.
Simple as the Dummy tells you!
The thing with this build is, you focus on assisting your allies with your Q (Ranged attack) and your heavy melee (auto attack). In fights the idea is to stay on the 2th or 3th place of entering on the moment of team fights. Your goal is to place your Ultimate attack on the way of your enemey so you'll slow / damage them. While your tank rushes in to make sure you and your dps can nuke the down, one my one.
Once I got more time I'll try to fix a better 'Written Guide'.