As Sharpshooter, you are focusing on getting headshots. Whether on smaller zeds or big zeds. If you have the proper weapons, focus on the bigger zeds because they are a bigger threat, they are rather easy to kill with headshots with some weapons, use pistol on trash zeds, always go for headshots. Its always important to get stronger weapons other than pistols so you can take down scrakes and fleshpounds when they start to spawn, such as the crossbow which is very effective when getting a headshot.

The weapons you want to get that will last you through the waves are the different kind of weapons that holds ammo.

In my case, I would have a crossbow, handcannon pistol, magnum pistol (or MK23 depending which you are comfortable with), and default pistol. When you have all these weapons, you will be able to last an entire wave on Hell on Earth. Its extremely useful because you can keep shooting. In this case from the game, you will have many bloody shooters!

When you are lower level, you will have to work your way getting these weapons. When you get them all, you are good to go especially against the Patriarch. At the boss battle, just keep your distance from the Patriarch and shoot him. Headshots will always be important as thats the main purpose of being the sharpshooter.

When hipfiring, the crosshair is always the center of the screen. You can actually see it yourself when shooting. It is the same as you aim down sight. This helps when fighting zeds at close range.

As max level sharpshooter, you will start with a crossbow. A really good head start for later waves and you wont have to worry about spending it because you already have it. Remember also to use crossbows on bigger zeds ONLY. Unless if thats your only weapon, you can penetrate and the bolts can be obtainable (though its hard to see at times, just remember where you shoot it and go get it, it will show that you obtain the crossbow bolt. Something you can make use of if you are conserving ammo).

Crossbow can hold off its own against Scrakes and Fleshpounds. In lower difficulties its almost always one shot in the head with it. At times, can stun scrakes. Which gives you a chance to try again before they snap out of it. Bolts are obtainable, though can be hard to see. The ammo is a bit expensive but its not really that bad. Just remember to use crossbow mainly on scrakes and fleshpounds. The rest you stick with pistols.

BOTH of them are actually good choices depending on how you use them. Both have their pros and cons so i'll try to explain how they work.

Magnum

Pro:
Has alot more power than MK23.
Devastating headshots, can be useful against Husks and Sirens.
Fire rate is much faster than handcannon.

Cons:
You will have a hard time aiming, including shooting. Because the way the magnum kicks back will make it difficulty for rapid fire, especially for headshots. A great marksman would know where to aim and shoot. New people or people that are still getting used to it or forgot the feeling out of it will have a hard time with it at times.

Poor reloading speed (However, when reached a good enough level it has a fairly good reload).

Less ammo than MK23, it holds 6 shots. You'll have to make use of those 6 shots before you reload.

MK23

Pro:
More accurate than Magnum, you can aim better even from a further distance against trash zeds.

Holds more ammo, 12 shots is enough to make your way through trash zeds. Not to mention last more longer.

Cons:
It is weaker than Magnum, will be tough to kill Husks and Sirens with it even with headshots. Takes more shots to kill them which means more ammo to waste.

Both pistols are useful regardless of their stats. You just have to make use of the shots. Headshots are the priority when using any of them.

Its a really useful pistol and very powerful. Despite its low fire rate it helps you concentrate better on getting headshots out of it. This even helps me when hip firing. I prefer you get a single handcannon in my opinion, because you can make use of the ammo and more space for other weapons. This weapon is more accurate than magnum, at least to me anyways since it helped me get a better steady shot. I tend to buy this first after crossbow (though since I am at max level, I would get handcannon first in this case because I already have crossbow).

This perk will help you last longer through the waves, possibly 2nd best out of all perks. First would be Berserker as it doesnt require ammo (except buzzsaw bow, but still most of the weapons are melee) in my opinion. Of course medic and other perks are as important too, so long the players know how to use it responsibility. As of you, since as sharpshooter remember to focus on scrakes and fleshpounds if you see any of them. If you do, take out your crossbow and go for headshots. Let your team handle the rest of the zeds. Fleshpounds will rage from the headshot, so shoot them in the head quickly before they come and attack. Scrakes will always be stunned with headshots.

I am aware some of you just wanted to find a point with smaller details so here it is!

http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1268844723

Follow this picture and get the weapons. 3 pistols you see there is handcannon, magnum, and default pistol. The 3 pistols I mentioned above through the details. MK23 is optional, I chose magnum. Both are good depending how you use them.

NOTE: If you are gonna ask me in regards of the total amount of dosh I had, I turned on mutator. Mainly because my point of showing this picture is to show what you needed in order to win. Its also not too hard to get it anyways on max level since the pistols are cheap too. Can be easily obtained. This picture I just posted recently and did the editing so you will have to pardon that.

You also start with a crossbow at the start of the game when you are max level, which is useful in starting round and gives you a better start when gathering weapons from my preference.

But with that aside, this will help your chances to survive. As long as your team are also doing their part and not have you to carry them.