As long as you have the Balloonicorn or the Balloonicorpse, you can get the achievement alone with commands.
Be sure to equip the item on your loadout. For this example, we'll use the Pyro.
Step 1: Start by loading the map "itemtest" using the "Create server" menu or with the following game command:
This map is very small and enemies will spawn around you.
Choose Blu team, then Pyro with the Balloonicorn.
Step 2: Disable respawn time with this command:
Step 3: Create an enemy Pyro using the following command:
sv_cheats 1; bot -team red -class pyro; sv_cheats 0
Step 4: To make the bot have the same loadout than you, type the following command:
sv_cheats 1;bot_mirror bot01;sv_cheats 0
(where Bot01 is the name of the bot)
Now proceed to kill the bot with the Balloonicorn. After he respawn, you have to repeat the Step 4 to reapply your loadout on him.
To get the achievement faster, you can repeat the Step 3 to spawn more enemies. You'll have to edit the command on Step 4 and change the bot's name.
Or group the commands like this:
sv_cheats 1;bot_mirror bot01;bot_mirror bot02;bot_mirror bot03;bot_mirror bot04;bot_mirror bot05;sv_cheats 0
Some bots will stay spectator until you execute this command: