Shrapnel - Rains down shrapnel on an AoE. Slows and does damage to Creeps, Heroes, and Buildings as well as giving sight in that area.
Headshot - Gives you a 40% Chance to do a mini stun and bonus damage on regular shots
Take Aim - Increases your range with every rank, up to 300
Assassinate - Lock on to an enemy Hero in sight and, after 1.7 seconds, fire a big, awesome shot at them which is NOT dodgeable!
In this build, prioritize your skills this way: Max out Take Aim, then Headshot, then Assassinate, put one rank in Shrapnel and then just put the rest into skills until 22. Obviously you have to put the last 3 skill points in Shrapnel.
NOTE: I may have screwed up the skill progression where it would tell you to put a point in Headshot when you can put one in Take Aim. This also somewhat comes down to preference and what feels right for the current situation though.
Maxing out Take Aim will make it so you can hang back behind your creeps and more durable heroes and still attack enemies. Sniper is pretty squishy.
Headshot's Crit chance will help a lot with Last Hits.
Assassinate is used for when a hero is going to get away but you can still see them. Or just use it whenever, at least at low levels. When it gets up in rank, the Mana cost gets pretty steep.
Shrapnel will be useful for getting sight on enemy Heroes further out to get your Assassinate off and when you want to see up stairs to attack a tower from a distance. The slow isn't exactly enough to do anything about Heroes getting close to you.
You will want to get mid. The Gold and Exp advantage helps to get this build going into full swing in a timely manner. If you feel comfortable not getting mid or you don't like going mid, this might take a little longer but the end result will be the same. Just make sure you get fed.
Circlet and Slippers - Turned into one of your Wraith Bands. These are good because they give you extra agility and damage and they are fairly cheap.
Branches - These can be sold after getting your second Wraith Band because, at that point, they are just taking up space for you to be building other items with.
Tangos - A Salve isn't usually needed because you will out range the enemy hero. Even better if they are melee but melee heroes in mid are somewhat scarce. Tangos will do the job just fine.
Just get into lane, harass the **** out of the enemy hero, Last Hit, Deny and take down the towers as fast as you can. All are good for getting a lead on gold and exp.
Try not to get pushed out of lane at all. Gank when needed because Hero kills will get you up on Gold and Exp.
Grouping will obviously start in the mid game. Just try to establish yourself as the carry and make sure you get fed.
Power Treads - These are just a standard part of any build. Not much explanation needed there. Just make sure they are set to Agility
Crystalis - This will give you more chances to Crit outside of your Headshot skill. Sadly they do not stack but more Crit chance is always nice.
Yasha - Agility, Attack Speed and Movement Speed. Nuff said
Depending on how much you get fed, this could apply to your mid game, and if it does, it will turn your mid game into late game.
NOTE: Which one of these two items you build first is up to you. It depends on how comfortable you are with using the Manta Style clones or which one seems better for the current situation.
Manta Style - Your Yasha will be upgraded into a Manta Style. Manta Style ups the passive agility to 26 and adds 10 Int and Strength. The active ability creates 2 clones of yourself that deal less damage and take more damage but they are still useful for doing more damage than normal and also getting away.
Desolator - This is the icing on top for this build. Besides the +60 Damage, when you shoot an enemy Hero, Creep, or Building, it will reduce their armor by 7 for 15 seconds. Sadly it does NOT stack, even if somebody else with a Desolator hits them but it still gives a HUGE boost in damage output, and combined with your illusions from Manta Style, you will be killing Carry and Durable Heroes in only a few shots
Get the Desolator and use it with your Manta Style illusions and you will be camped outside their fountain in no time, just picking them off as they come out of the fountain. Even if they don't leave the fountain, your range will let you fire at them while they are in there and your damage output will be enough to overpower the healing in the fountain.
For your Luxury Items:
Shadow Blade - This is really only in there to up your Damage and Attack Speed. The effect isn't exactly needed because usually by the point you will be able to get it, you will be camping their fountain.
Dadedalus - The progression of the Crystalis and will just enhance its effects, which is always nice.
Sange and Yasha - Even though the Movement Speed bonus doesn't stack with the Manta Style, will up your Attack Speed, Agility, Strength and Damage even more.