Sniper

Sniper being the most straightforward class (Find a spot, aim, BOOM HEADSHOT), this post will be relatively short and will only deal with the weakness of the sniper, close combat, and the unlocks.

GENERAL STRATEGY

Other than the progression mentioned above, Sniper has only one other thing to consider: close combat.

Close combat is fought by a Sniper as a means of getting away, not of actually trying to kill. You want to hold him off until a combat-oriented class can come over and finish him off. Let your team know you need help (Either through voice chat or voice commands) and (hopefully) they will help you.

Jarate makes the Sniper a more support-ish support class by giving him an aspect beyond that of hiding far away and shooting people every once in a while. You can put out fire, which makes Arena matches infinitely easier, and you make enemies take more damage, making winning said matches easier.

UNLOCKS

Hunstman (First Unlock, replaces Sniper Rifle) - Makes Sniper a viable mid-range combat class. Useful for Arena, where there are few Sniper nests. However, makes long-range combat difficult. Use discretion.

Razorback (Third Unlock, replaces SMG) - Don't ever use this. The benefit (blocking one backstab) isn't useful in most situations versus the detriment (slower speed + loss of the SMG).

Jarate (Second Unlock, replaces SMG) - Useful if you're using the Hunstman, as you're in the thick of combat already. If you like to camp far away from the rest of your team, use SMG as your out if it gets back. Otherwise, if you're with your team, use Jarate.