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.
This is the build I use to dominate middle lane. It builds fast and you should be harrassing your opponent early and often. Between 6-8lvl is where I usually score a kill!
So Magic Penetration, Ability Power are your priority as you level. Most players don't realize how my Magic Pen really hurts, and most dps do not get a Magic Resist item.
Basically at the start throw barrels that can hit your opponent as well as nearby mobs, once you get it to 3rd or fourth level the Barrel Roll starts to hurt... a lot! Pace your skill use so that you will always have mana to throw a Barrel Roll. Remember priority is to kill minions tossing an occasional Barrel Roll at your opponent when they aren't near minions to harrass is fun but more difficult.
Sixth level is the magic level. This is where combos are key. Basically I move forward and melee mobs click the targeter for Explosive Cask and wait for them to commit. I then throw the barrel over their head which in turn bounces them toward me I then Body Slam and Barrel Roll, melee them to death.
I have no sixth item, that is your choice it is dependent on what you need, if your team is good keeping everyone away from you then go dps with a Zhonya's or a Tank item if you find yourself getting really punished.