From the [Obtaining Demoman Achievements][7] page on the [official TF2 wiki][1]:

> Left 4 Heads
> ------------

> Decapitate 4 players with only 10 seconds between each kill.

> **Difficulty:** Hard  
> **How to obtain:** Equip the [Eyelander][8], [Scotsman's Skullcutter][9], [Horseless Headless Horsemann's Headtaker][10], [Claidheamh Mòr][11], or [Half-Zatoichi][12] and engage multiple enemies at close range. Pair your melee weapon with the [Chargin’ Targe][13] in order to reach new targets before the ten seconds expire.


  [1]: http://wiki.teamfortress.com
  [2]: http://wiki.teamfortress.com/wiki/Scotsman%27s_Skullcutter
  [3]: http://wiki.teamfortress.com/wiki/Claidheamh_M%C3%B2r
  [4]: http://wiki.teamfortress.com/wiki/Half-Zatoichi
  [5]: http://wiki.teamfortress.com/wiki/Eyelander
  [6]: http://wiki.teamfortress.com/wiki/Horseless_Headless_Horsemann%27s_Headtaker
  [7]: http://wiki.teamfortress.com/wiki/Obtaining_Demoman_achievements
  [8]: http://wiki.teamfortress.com/wiki/Eyelander
  [9]: http://wiki.teamfortress.com/wiki/Scotsman%27s_Skullcutter
  [10]: http://wiki.teamfortress.com/wiki/Horseless_Headless_Horsemann%27s_Headtaker
  [11]: http://wiki.teamfortress.com/wiki/Claidheamh_M%C3%B2r
  [12]: http://wiki.teamfortress.com/wiki/Half-Zatoichi
  [13]: http://wiki.teamfortress.com/wiki/Chargin%E2%80%99_Targe