If you have trouble getting this achievement legitimately, you can unlock it in singleplayer with some commands.
Step 1: Load the map "itemtest" using the "Create server" menu or using the following command:
map itemtest
Choose the BLU team
Step 2: Disable the unbalance limit:
mp_autoteambalance 0
mp_teams_unbalance_limit 0
Step 3: Create the bots using these commands:
sv_cheats 1;bot -team red -class scout;sv_cheats 0
sv_cheats 1;bot -team red -class soldier;sv_cheats 0
sv_cheats 1;bot -team red -class pyro;sv_cheats 0
sv_cheats 1;bot -team red -class demoman;sv_cheats 0
sv_cheats 1;bot -team red -class heavy;sv_cheats 0
sv_cheats 1;bot -team red -class engineer;sv_cheats 0
sv_cheats 1;bot -team red -class medic;sv_cheats 0
sv_cheats 1;bot -team red -class sniper;sv_cheats 0
sv_cheats 1;bot -team red -class spy;sv_cheats 0
Step 4: Kill them all!
Have fun!