The following is a full list of the 35 Sniper-specific achievements for Team Fortress 2 that make up the “Sniper Pack”. Although not containing Milestones and unlockables on release, Valve has since re-included these following intense criticism of the “item drop” system that was meant to replace them. Below you’ll find the name, icon (click for full size), description and a difficulty rating on a scale of 1 – 4 for each achievement.
Righto then, mate:
The 35 new Sniper Achievements are as follows:

Australian Rules
Dominate an enemy Sniper.
Difficulty: 2

Be Efficient
Get 3 kills with the Sniper Rifle without missing a shot.
Difficulty: 2

Be Polite
Provide an enemy with a freezecam shot of you doffing your hat.
Difficulty: 1

Beaux And Arrows
Kill a Heavy & Medic pair with the bow.
Difficulty: 3

Consolation Prize
Get backstabbed 50 times.
Difficulty: 1

De-Sentry-Lized
Destroy 3 Engineer sentry guns.
Difficulty: 1

Dead Reckoning
Kill an enemy with an arrow while you’re dead.
Difficulty: 2

Drop Dead
Kill a Scout in midair with your Sniper Rifle or the Huntsman.
Difficulty: 2

Enemy At The Gate
Kill an opponent within the first second of the round.
Difficulty: 1

Friendship Is Golden
Extinguish a burning teammate with your Jarate.
Difficulty: 1

Have A Plan
Capture the flag in CTF.
Difficulty: 3

Jarate Chop
Jarate and then kill 3 enemies with your Kukri.
Difficulty: 2

Jarring Transition
Use Jarate to reveal a cloaked Spy.
Difficulty: 1

Jumper Stumper
Kill a rocket or grenade-jumping enemy in midair with your Sniper Rifle or the Huntsman.
Difficulty: 2

Kill Everyone You Meet
Kill 1000 enemies.
Difficulty: 3

Kook The Spook
Kill 10 Spies with your Kukri.
Difficulty: 2

My Brilliant Career
Top the scoreboard 10 times on teams of 6 or more players.
Difficulty: 4

Not A Crazed Gunman Dad
In a single life, kill 3 enemies while they achieving an objective.
Difficulty: 2

Number One Assistant
Score 5 Assists with the Jarate in a single round.
Difficulty: 2

Parting Shot
Headshot an enemy player the moment his invulnerability wears off.
Difficulty: 1

Pincushion
Hit an enemy with 3 arrows, without killing them.
Difficulty: 2

Rain On Their Parade
Jarate an enemy and the Medic healing him.
Difficulty: 1

Robbin Hood
Take down an intelligence carrier with a single arrow.
Difficulty: 2

Rode Hard Put Away Wet
Jarate an enemy that you’re dominating.
Difficulty: 2

Saturation Bombing
Jarate 4 enemy players with a single throw.
Difficulty: 2

Selfdestruct Sequence
Headshot 10 enemy Snipers.
Difficulty: 2

Shafted
Stab an enemy with an arrow.
Difficulty: 3

Shock Treatment
Kill a Spy whose backstab attempt was blocked by your Razorback.
Difficulty: 2

Shoot The Breeze
Kill a fully invisible Spy in a single hit.
Difficulty: 2

Socket To Him
Headshot an enemy Demoman.
Difficulty: 1

The Last Wave
Provide an enemy with a freezecam shot of you waving to them.
Difficulty: 1

Triple Prey
In a single round, kill the same enemy with 3 different weapons.
Difficulty: 4

Trust Your Feelings
Get 5 kills with the Sniper Rifle without your scope.
Difficulty: 1

Uberectomy
Kill a Medic that is ready to deploy an ÜberCharge.
Difficulty: 2

William Tell Overkill
Pin an enemy Heavy to the wall via his head.
Difficulty: 2