Tips to Use Phantom Assasin (Dota2)

• Stifling Dagger has multiple uses. It should generally be the first skill maxed, as it allows you to last hit creeps even if they are far from you and later on allows you to slow enemies at a great range.
  • Avoid using Stifling Dagger as a harass tool in the laning phase before level 5, as you'll be losing a good amount of CS and it won't deal enough damage to ward off your enemy. However, you should use it on enemy heroes when they back up and attempt to use non-combat items such as Clarity and Healing Salve.
  • Depending on your team's strategy, Phantom Strike can be taken at level 1 as the increased DPS combined with allied disables can secure First Blood.

• Phantom Strike can target allies and enemies. Jumping to a friendly unit can be useful if Mortred needs to escape.

• Leveling Blur is usually not recommended early on. However, a single point can be very helpful if your lane opponents are primarily harassing you with basic attacks, or orb-walking.

• There is a temptation among unseasoned players to focus on the maximisation of attack damage, neglecting attack speed. However, focusing on attack speed generally produces more favourable results; her critical damage is gigantic even with low attack damage and high attack speed means she is much more likely to kill her target, as opposed to relying on huge-but-infrequent criticals. In essence, the faster her attack speed; the less likely you will fail to kill a target because of lack of luck; and three Coups de Graces with average base attack are much stronger than a single one with high base attack.

• However it is not advised to build Mask of Madness on her for increased attack speed in a similar manner that it is built on heroes with bash as Phantom Assassin has very poor health and has no means to disable her opponents from attacking her while she is more vulnerable whilst Faceless Void has Chronosphere that freezes his enemies and Spirit Breaker's Greater Bash, Charge of Darkness and Nether Strike prevent retaliation.

• Be very wary of enemies using Blade Mail, as Mortred's damage output is many times greater than her health pool, though with a Black King Bar you can alleviate the issue if the enemy does build a Blade Mail.

• In the laning Phase of the game sometimes if you are facing severe harassment and your team is incapable of giving you the space to farm your items you might want to consider building a Mekanism on your hero as her skills hardly use any mana and because of her low health pool Mortred can get up to 1/3 of her health back early on and contribute to her team earlier. This item gives you more survivability against both nukes and physical attacks. Though building a Mekanism is VERY situational on a hard carry. 

source: http://dota2.gamepedia.com/Phantom_Assassin