In all honesty, you could get this achievement in a public server if you're good enough at Spy/have enough luck to catch a heavy-medic duo off-guard.

However, if you're like me, you lack that skill and luck, so you want to earn this achievement in a slightly more creative way.

From the main menu:

• Enter 'map itemtest' into the console
  
• After the map loads, join as BLU Spy
  
• Disguise as any RED class

Once that's done, do the following:

• Enter 'mp_autoteambalance 0' into the console
  
• Enter 'mp_teams_unbalance_limit 0' into the console
  
• Enter 'tf_bot_add red medic'
  
• Enter 'tf_bot_add red heavyweapons'

Now you should have a RED Medic and Heavy next to each other. They won't move around and will stay wherever they spawned.

If the Medic is out of range and doesn't start to heal the Heavy, try using 'tf_bot_kick [bot name]' and then using the above 'add' command to spawn another bot. Hopefully the new bot will spawn close enough to start the healing procedure.

Getting the achievement is very simple.

• Make sure the medic is healing the Heavy
  
• Backstab the Medic
  
• Quickly get behind/circle around the Heavy and backstab him

If you do all this within ten seconds (and without dying to the Heavy), then you should have your achievement.

Best of luck!

~

P.S. You can also use this method to unlock the Scout-related Back 2 Back achievement, it just takes some minor tweaking!