Obtaining Spy achievements
From TF2 Wiki
Kill 50 enemies with the Ambassador.
How to: Not necessarily difficult, but may take a long time, even moreso since the Ambassador does less damage than the standard Revolver. You can attempt this by headshotting unwary Snipers with your Ambassador (which will also count towards the "Skullpluggery" achievement) rather than backstabbing them, though in all cases it will take two or more shots unless they were wounded previously.
|Is It Safe?
Backstab 50 enemies who are capturing control points.
How to: This is a grind achievement, but a more difficult one. Some control points are very confined, making it difficult to walk in without bumping into somebody. Also, more skilled players tend to Spy check anyone who comes in. A lot easier on CPs that are in a more open area. This may be easier to accomplish by camping a sparsely defended control point (such as in Gravel Pit) with the Cloak and Dagger, and then eliminating anyone who approaches to capture it. It can, however, be considerably easier to perform on Payload maps, where players huddle around the cart and shoot at your teammates, not paying attention to their backs.
Destroy 1000 Engineer buildings with sappers.
How to: Will happen over time. The best way to get this achievement is to kill Engineers before sapping their buildings, as Teleporter Entrances can be saved from the other side, though you can avoid this by repeatedly sapping the Exit after they knock your sapper off. Beware of servers with instant spawn though, as those will allow an Engineer to almost immediately teleport back to his buildings to knock your sappers off. If you can get to the enemy base undetected, then sapping the Teleporters and telefraging the Engineer will make this significantly easier.
Kill an enemy who triggered your feign death in the last 20 seconds.
How to: Most enemies will not be expecting you to backstab them after they believe you're dead. If it fails and/or the enemy starts getting suspicious, try fighting him first until he starts to take away some of your HP. Then quickly pull out the Dead Ringer before he gets the next hit in, cloak and get behind him for the backstab. You can also try fooling people by intentionally playing sloppy with your Dead Ringer out, then get behind them after they attack you. Note, however, that the Dead Ringer makes a very distinct sound when uncloaking, so you may wish to uncloak farther away and then finding your target using the remaining time.
Backstab 1000 enemies.
How to: Should happen over time. This achievement will come faster if you backstab people in groups, such as when they're huddled around the payload cart or are getting ready to make a push.
|The Man with the Broken Guns
Backstab an Engineer, then sap 3 of his buildings within 10 seconds.
How to: This achievement is much easier now due to the August 13, 2009 Patch. Simply find an Engineer with a building next to an active teleporter, then backstab him and sap the building and the teleporter. You will automatically place another sapper at the other end of the teleporter. It is not necessary to destroy the buildings with your sappers, simply for sappers to be placed on three buildings belonging to the Engineer that you backstabbed. Knowing how to "stab and sap" will help if the building is a Sentry gun.
|Who's Your Daddy?
Headshot 3 Scouts with the Ambassador.
How to: Scouts move quickly and are hard to headshot if you do not have good aim. Unless they are standing still (ie. trying to hit a target with their Sandman, attempting to kill someone with their taunt, or are sitting near a dispenser), this one is largely luck. On the flip side, you may occasionally get a lucky shot if being chased, and due to their low HP a headshot will usually kill Scouts if they are not at full health, especially those carrying the Sandman.