This is one of those achievements you're unlikely to get nowadays without asking someone for help or seeking out ironic Pyrovision/Balloonicorn players on 2Fort.

This guide should allow you to get the achievement in a matter of minutes, but you will need some things first.

Before you begin, you will need the following:

• Pyrovision Goggles (can be found for like £0.10 on Community Market)
  
• Balloonicorn/Balloonicorpse/Reindoonicorn

If you don't have any of the Balloonicorn cosmetics, just go to a community trading website where you can usually get the original Balloonicorn for less than two refined metal.

Failing that, you can just craft the Balloonicorn, if you can really be bothered.

From the main menu, equip the Balloonicorn and Pyrovision Goggles on your Spy loadout. Once you have those items equipped on Spy, let's start a game.

• On the main menu, type 'map itemtest' into the console
  
• Press enter and let it load
  
• Once the game loads, join BLU and spawn as a spy

Now, you'll want to enter these commands into the console:

• 'mp_autoteambalance 0'
  
• 'mp_teams_unbalance_limit 0'
  
• 'mp_respawnwavetime 0.5'
  
• 'tf_bot_force_class spy'
  
• 'tf_bot_add red spy 10' (adds ten RED spies - Spy can't attack on the itemtest map)

Now you will have ten idle spies on the opposite team. All you need to do is follow the steps below and repeat about 7-10 times.

• Enter 'sv_cheats 1; bot_mirror all; sv_cheats 0' into the console
  
• The enemy spies will be disguised, but they will now have your weapons and cosmetics
  
• Kill all the spies (once they respawn, they go back to stock)
  
• Once you kill each spy, simply re-enter the above command and repeat

After doing this for a few minutes, you should have your achievement. Just don't waste time killing spies that have respawned but haven't been re-equipped with your loadout/cosmetics.

Happy achieving!