Open the command console: ESC > Gear Icon > Under the keyboard tab select Advanced > Check off Enable Developer Console
Press the ` button to open the console and type the command below: (This was the Map i used, you could swap it with any Capture the point map)
map sd_doomsday_event; hostname "boost"; sv_lan 0; sv_cheats 0;
That will create your match. Choose the Red Team. Then type the following command:
mp_teams_unbalance_limit 0;
This will allow you to stack the opposite team. Then add a bots using the command:
tf_bot_add blue;
You will also want to use the Command
tf_bot_melee_Only 1;
This will stop the bots from firing. They wont really move too much until you get too close or attack. Then they will just sort of run back and forth or run away while meleeing.
Now I unlocked mine when everyone was teleported together for the curse. I was able to throw the Jarate and get 4 of them. Another option would be when the bots are going for Capture points. You could also combine this with Dummy accounts if you have some.
NOTE: Some Achievements wont unlock unless there are a certain amount of players\bots in the game. I cant recall if this one was one of them. If it does not unlock, make sure you make the game 6 Vs 6.