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.
The skill sequence is pretty straight forward. If your last hitting well your Bio-Arcane Barrage should be maxed right around when you can afford your blood razor and this is where the bulk of your damage comes in. Also Caustic Spittle takes priority over Void Ooze because of the passive attack speed it offers and armor pen.
I chose ghost and exhaust because this build can be fairly weak early on til you get your blood razor and the two work tell together for dodging ganks early on and chasing later. I would imagine flash being viable as well but I just prefer ghost especially since you can get the mastery for it without specing to far down.
Pros / Cons
stupid amounts of damage when fully set up
Weak early on
Need to be good at last hitting or early ganks
The only really arguable item in this set is the gunblade and especially getting it before last whisper. The reasoning is because it has both spell vamp and life leech. This is helpful because his damage is split into both physical and magic so this is the only effective way to get leaching. The extra ability power on gunblade is also just enough to bump Bio-Arcane Barrage up from 6% to 7% and give his ult a little more damage. The 300dmg active with a slow is also nice to have if you don't have your frozen mallet yet or exhaust is on cooldown.
Also, for boots I only added in Berserkers for the sake of the purchase order. Those are the default boots I buy but they also vary depending on the other team. If your team has a lot of Magic damage or slows get Mercury treads or if they have a good AD carry get Ninja Tabi. Kind of self explanatory...
No dedicated defensive item, this is because I don't find it necessary with this build. With the high movement speed and ability to slow it is easy to escape. Also with the massive leaching and 700hp from frozen mallet it is easy to survive team fights if you keep attacking. Another aspect of this is the fact that 1v1 is almost an instant win because after he is geared they have about 4-5 seconds to kill you no matter their hp because it is all percent based. They also cannot run or try poking because of your ult and the massive range buff Bio-Arcane Barrage gives.