The Spy is another class in which the Mann vs Machine game mode is a great way to learn how to play it, but the class itself isn’t that helpful to he team. While playing thee were even point where teammates wanted to kick me just for playing as the Spy. This changes in the later waves, but only if the player has all the cash from the earlier waves for upgrades. A good strategy to solve this problem might be to play the first few waves as another class, with out purchasing any upgrades. Then in the later waves switch o the Spy, and purchasing all the upgrades you need.
An essential part of the Spy’s play is his ability to disguise. For some reason the cloak watch isn’t available in mvm, so disguises are the only stealth mechanic available. Mvm is a great game mode for learning the basics of disguising, as before each wave the player is told what classes they will be facing so the player can known what class to pretend to be. Also the AI of the robots aren’t so smart that they’ll figure out who you are if you don’ act a certain way. Even when our sapping their team, they probably won’t attack you.
The Spy’s most useful tool in this game type is the sapper. In mvm the sapper will cause the targeted enemy robot to freeze for some time, as well as any other robots with a certain range of the first. This is essentially the only thing the Spy can do against enemy scouts, as they are too fast to back stab, too numerous to melee or shoot. For this reason it is a good measure to fully upgrade the sapper when you can.
The next best asset to the Spy is his back stabs, especially against the giants. Against Heavys is when the spy is the most useful. The Spy can quick kill this class, which would normally give any other player a lot of grief. This strategy work against the giants as well, though it is not an instant kill, but can do a lot of damage. The giants are usually so cumbersome that they won’ be able to turn around until after the player has gotten a few more knife strikes after the initial back stab. For this ability it is a great idea to also purchase the armor piercing upgrade for the Spy’s knife, so this back stab against giants will do more damage.
Article from Gamersyndrome.com