Spy

One of the most difficult classes to play, Spy is less point, click, then shoot; and more about infiltration than anything. Spies have one of the only instant-kills in the game, the backstab, and have the only item designed specifically against one class, the sapper. This makes them highly specialized.

GENERAL STRATEGY

As a spy, trust your cloak, not your disguise. When you're cloaked, nobody can see you. When you're disguised, people can monitor your behavior and spycheck you. If you bump into anyone while disguised, you will be immediately shot, whereas if you're cloaked, you can get away and have them shoot wildly. They will be on full alert, true, but it will pass if you are patient. This is the number one skill of successful spies. Be patient. Don't go for a risky backstab immediately, wait until it becomes more clear. Don't sap while the Engineer is working, wait until he goes and gets metal.

The old debate goes: sap then stab, or stab then sap? Do you sap the Engineer's buildings then kill him, or take care of him first? The answer lies within a couple of factors. First, consider the damage caused by the sentry once you undisguise yourself. If it's level one, or distracted by someone else, you should be fine. Second, consider the knockback. If you're knocked back by the sentry after you kill the Engineer, you're dead, as you can't approach the sentry to sap it. Overall, it's better to sap then stab.

Engineers will have two responses to your saps: either unsap, or try and kill you. They will undoubtedly know you are a spy, so they will react most definitely in one of these two ways. If he tries to unsap, keep applying the sapper. You have an unlimited supply, so you should be able to keep going until it dies. After a while, though, most Engineers will try and attack you with the wrench, so back off and shoot him to death.

Backstabbing is often called the primary of the Spy. If you're not trying to sap, you will be trying to kill via backstab. Nobody goes for the revolver kill. It is a useful tool for getting out of tough situations, but it's not the ultimate goal. Backstabbing should be for primarily slow enemies. Kill Heavies, Soldiers, and Snipers, mostly. Not only will this take out a huge part of any team's offense or defense, it will cause the rest of the team to focus on finding you which, if you're good, will take a while. At this point, your team can push, taking down the confused team. Second, backstabbing should only be done once you're sure. If you've managed to avoid enemies thus far, you can wait a couple more seconds. More times than not I've gone for the backstab and the enemy turned around right as I was trying to kill him.

AVOID PYROS AT ALL COSTS. If you walk near one and he even accidentally presses his Mouse 1 button, you're dead. Don't even try to backstab them, they will be constantly spychecking and therefore will kill you often.

UNLOCKS

Ambassador (First unlock, replaces Revolver) - Slight damage dropoff for accuracy (with a cooldown), and crits on a headshot. I would definitely use this. Wait for the cooldown if you can, so you can have pinpoint accuracy for the headshot crits, which can end any firefight relatively quickly.

Cloak and Dagger (Second unlock, replaces Watch) - Indefinately recharges while standing still but, while moving, runs down based on how far you have moved. I don't like this because I pick up metal on the battlefield to recharge my cloak (And to piss off enemy Engineers), but many people use it as a way to hide until the opportune moment is found. Use at your own discretion.

Dead Ringer (Third unlock, replaces Watch) - If you have it out, fakes your death if you recieve nonlethal damage and cloaks you for up to 8 seconds. I don't like this because, along with the inability to pick up metal for cloak, it makes an obnoxiously loud sound when uncloaking, and most people can figure out a Dead Ringer Spy as soon as they shoot.