Core Items

First Item Astral Catalyst should let you gank more often after hitting level 6. It reduces your Ultimate Cooldown by 20 which let you gank ->farm one side jngl or Fangtooth-> gank -> farm other side jungle -> gank -> and so on...

As glascannon you should look to take every bigger Damage spike you can get to get a faster clear in jungle and get those early lanesnacks.

As you get Oathkeeper you should technically oneshot every basic laner, that is not an bruiser or tank. Especially if you fill in your Spell rotation those nasty Auto Attacks!
That means Engage with Q -> Basicattack -> RMB -> R -> Basic attack -> 2 (Soulstealer Obelisk) -> E -> disengage with Q reactivation or run them down if they not died yet.

Situational changes

• Megacosm is an option if the Enemy Team has more than one Tank/bruiser. You should be able to 1v1 them after 4 Items, depending on theire state in the game. Trade this item in the Corebuild in for the Wraith leggings but make sure to buy it after your 2nd (questitem "obelisk" not involved) and before your 5th item!
• Combustion is the other option you have, if you see that half of the enemy team is stacking up this annoying Magic resistance. You should keep the wraith Leggings and trade combustion in for the Oathkeeper. You can build it after your 4th or 5th Item.

Playstyle

As Assassine your pretty much more like a glascannon. Very high DPS but not sustainable at all. Try to sneak up some nasty ganks when nobody expects them. Ping your laners to go in first and in Teamfights make sure the enemy team dont know where you are. Then attack from the flanks or behind and Remember to reactivate your Q for smooth in-and-out missions.

VERY IMPORTANT

• Trade in your useless ward for the lense! It helps so much by ganking and setting up Fangtooth/Orb Prime.
• Dont forget to use your Obelisk to collect those souls!