The game currently has two snipers and they work differently.
Widowmaker
Widowmaker has a grappling hook that she can use to pull herself up to ledges and rooftops. You should use this to position yourself somewhere where it's hard to see you.
Her alt-fire will use her zoom scope. When zoomed in, a power meter appears on your screen. When the power meter is full (takes about 2 seconds), headshots will do 300 damage and body shots will do 150 damage.
Keep in mind that a few characters have a different head position:
- D.va's head is...
- Mech: The entire cockpit she sits in.
- Girl: Her head
- Bastion's head is...
- Recon configuration: His head
- Sentry configuration: The glowing spot on the back of the turret near the top.
- Tank configuration: The glowing spot on the back of the tank near the top. (This used to be at the top of the tank, but I'm pretty sure that changed during beta.)
Bear in mind that death replays as well as your sniper rifle's tracers (the red lines that show where you were shooting) give away your position, so you periodically need to move.
Don't forget to set your venom mine down somewhere. It's best use is as an early warning that an enemy is nearby as it shows on your screen when it's triggered.
Your ultimate should most likely be used shortly after you get it as it shows all enemy positions to your team.
Hanzo
Unlike Widowmaker, Hanzo doesn't have a zoom scope. As such, you tend to do better when you're closer to an enemy. Constant movement between shots helps quite a bit.
Remember that you can climb walls and use this to your advantage.
Hanzo's arrow do more damage if you charge the bow for a second before letting go of the attack button.
You should use your Sonic Arrow whenever it's ready to help show your allies where enemy players are. Keep in mind that, unlike Widowmaker Ult, it only shows enemies near the arrow.
On Defense, consider shooting it at one of the enemy doors during setup time so that you can see what heroes the other team has.
Your Scatter Arrow is a mixed bag. It's most powerful when shot in a small room as the shots ricochet off walls. However, you can still do a lot of damage with it by shooting the floor directly in front of an enemy.
Your ultimate is incredibly powerful as it sends the twin dragons/wolves in a direct line in front of you and travels a good distance, but bear in mind that it won't hit anyone within about 5 meters of you.
All weapons with the exception of Pharah, McCree's secondary, Junkrat, Mei's primary, Reinhardt, Winston, Zarya, Ana (on friend or foe), and Symmetra can headshot. Damage reflected by Genji can headshot as well as the initial arrows for each Hanzo's abilities (though the scattered projectiles and dragon themselves cannot). Roadhog's Whole Hog ultimate can headshot. Other abilities and all melee attacks cannot headshot.
Best Answer
Other than breaking line of sight, there is no general way of "dodging" it. There are character specific skills you can use to avoid it.
Basically anything that can make you immune to projectile damage should work, if timed properly. Otherwise, your only option is to kill him first or break line of sight.