While cloaked, bump into an enemy cloaked Spy.
This achievement is a relatively easy achievement for the Spy, as it only requires both you and your target to be cloaked. Chances are you will get this achievement on accident, thanks to the close-quarters nature of combat in most maps. Simply run around cloaked. As soon as you bump into a cloaked enemy Spy, you will net the achievement. As is, there is no point to even farm it, as it should be one of the first achievements you get. If you really want to, though, it is easily farmed with Counter Espionage, as you can have your friend disguise as a friendly and let himself get backstabbed.
Note: Both spies must be fully cloaked in order for the achievement to trigger. If either of you are paritally cloaked, it will not count. This gives you approximately a ten second window (not counting picking up ammo to replenish your cloak) to get the achievement before your cloak must recharge.
Backstab 1000 enemies.
Another grind achievement, this has you performing the Spy's characteristic backstab maneuver 1000 times. Much like the other massive grind achievements, there is absolutely no point in farming this. We mean it. Just play the game. You will get credit towards this achievement when you do any of the backstab achievements, but there is no point in sitting there and backstabbing a friend 1000 times. That's just silly! If you need the new gear so bad, just get some of the other, less grinding achievements. We promise that if you just play the game and backstab people for this achievement, you will enjoy it more. After all, nobody likes to sit in one place for hours in a first-person shooter. That's strictly relegated to strategy and massively multiplayer online game territories!
The Man from P.U.N.C.T.U.R.E.
Stab an enemy while fencing.
Seems like everybody is getting one of these fancy kill taunts, doesn't it? The Spy's taunt with the knife is now a killing taunt much like the others, and has a decent range to boot. To get the achievement, head to the enemy balcony in 2fort. Assuming nobody is around, stand next to a Sniper and taunt with your knife equipped. With any luck, you'll stab him and net yourself the achievement. However, it does have a long start time, much like the other kill taunts, so be wary of other players. You can't stop mid-taunt, after all. Farming this achievement is much the same, except have a friend stand still for you. As this is a taunt achievement, you can not attempt it alongside any other achievements. This is the only achievement that will net you anything from the kill taunt.
Note: Note that it is not the first two stabs that will net you this achievement, but the third and final stab, which is an instant kill for anyone unlucky enough to be in the crossfire. Also, the achievement name is a not-so-subtle nod to the famous spy television series The Man From U.N.C.L.E.
The Man with the Broken Guns
Backstab an Engineer, then sap 3 of his buildings within 10 seconds.
A difficult achievement for the unaccomplished Spy, this one requires you to possibly expose yourself to a sentry gun before spaping it, which is always a dangerous state of affairs. The best way to do it is to backstab the engineer, strafe with the rotation of the turret, and plant the sapper. With the turret down, you can go ahead and sap the other nearby buildings. In order to get the achievement, you must sap any combination of the four buildings. For practical purposes, it will probably end up being sentry, dispenser, and teleporter exit.
To get this achievement through farming, have a friend build all four buildings in a single spot. Sap the sentry, backstab your friend, then sap the rest. You should net this achievement, Sapsucker, and credit towards the Sap Auteur achievement. This Sapsucker into The Man With The Broken Guns achievment chain is one of the only achievement chains in the game, so make good use of it, farming or otherwise.
Note: This achievement is an obvious allusion to The Man With The Golden Gun, one of the better known James Bond movies. It was the second movie to star Roger Moore, and lends its titular gun to Bond in the console game Goldeneye.
The Melbourne Supremacy
Dominate a Sniper.
For any decent Spy, this achievement will be as easy as walking. Make your way to the enemy Sniper positions and have a ball. The catch is that the Sniper can't kill you at any point, making it necessary to get the drop on them every single time. If you manage to do this several times without dying, you will net yourself a domination and the achievement. Alternatively, instead of backstabs, you can get headshots with the Ambassador. Good luck with that, though. Trying to out headshot a Sniper is like playing chicken with a nuclear bomb.
To farm this achievement, have your friend stand still while you stab him over and over until he is dominated. Instant achievement! Goes best with Die Another Way, Skullpluggery, and Identity Theft, for obvious reasons. Keep in mind that you can always let him kill you afterward for one of his achievements. It's only fair, after all!
Note: You can be dominating the Sniper through any method, including assists. The name of this achievement is a nod to the book (and movie adaptation) The Bourne Supremacy by Robert Ludlum, which is the second in the highly-regarded Bourne spy fiction series.