In my opinion if you want to kill easily a dominating sniper I would suggest use the Spy equipped with Ambassador.
This give you the chance to avoid backstabbing him, if wearing Razorback, and hit directly with a nice headshot. Headshot + single body shot is a sure kill.
If you are not an expert spy or the situation around the sniper is really caotic, you can equip Cloak & Dagger so you can hide in a safe corner and wait the right moment to get your revenge.
He will call you lamer, but this is TF2, a rock paper scissors game where all classes have their nemesis (or maybe more than one).