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.
in teamfights use your "r" to stun all the enemies. its a good idea to stack your pasive up with your "e" its cheap in mana and also serves you good if you will be attacket.
After that use your q,w,e (if its off). if they aint dead by now flash into them and do q again and ignite.