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.
Ignite : Always a good extra damage output.
Ghost : Good for escapes / catching up.
Exhaust : Kinda useless later in the game with this type of build.
Flash : Really awesome to get close (surprise!) and shoot of hextech gunblade.
Teleport : Is fine when playing 5v5 (never uses it myself) but having combat / escape spells . gives you the edge in team fights etc.
I max mark of the assassin first (except shadow dance) since its a good harasser and farmer but i always have all my spells (cresent slash for farming and twilight shroud for escape / stealth)
after that i up each every lvl.