You have to remember that a lot of his power is contained in his stealth.
If you are mid, stealth really doesn't do much for you. Actually when laning in general, stealth is weak and thus so are you.
Fortunately, the stealth ability also has the tasty Attack Speed boost so it is still usable when laning. You will stealth, then move to the side of your target and let loose. Get in about 2 or 3 shots and whether they run or move to attack, you scoot so you don't eat Minion damage. Then Expunge em for some bonus damage. Repeat.
I like maxing expunge early (or at least take 3 points) because this is the ONLY time it is useful! It scales poorly so either use it early or skip it.
Assuming you can get in 3 attacks (which is doable due to AS boost and stealth), it hits for 90, 150, 210, 270, 330 which is pretty strong for such a long range autohit nuke.
1 rank in slow is good enough and gives you 2 seconds duration. More ranks ups the duration ONLY and nothing else.
Your innate poison is pretty good too. It basically adds 15 damage over time every shot, and increases every 5th level or so.
So, as long as you position yourself to not take undue creep damage, you can outshoot Ashe and the like. +30% AS, +15 dmg poison, and a large nuke ensure you will win. Creeps are your worst enemy since their damage adds up fast.
Since you will trade damage, I like the Regrowth Pendant -> Philo Stone. Lots of health regen and plenty of mana for the whole game. Extra gold is nice too.