From the Obtaining Demoman Achievements page on the official TF2 wiki:
Left 4 Heads
Decapitate 4 players with only 10 seconds between each kill.
Difficulty: Hard
How to obtain: Equip the Eyelander, Scotsman's Skullcutter, Horseless Headless Horsemann's Headtaker, Claidheamh Mòr, or Half-Zatoichi and engage multiple enemies at close range. Pair your melee weapon with the Chargin’ Targe in order to reach new targets before the ten seconds expire.