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:
map sd_doomsday_event; hostname "boost"; sv_lan 0; sv_cheats 0; (I had to input this twice to be detected on my LAN for some reason)
That will create your match. Choose the Red Team. Then type the following command:
mp_teams_unbalance_limit 0; mp_idledealmethod 0; mp_idlemaxtime 0;
This will allow you to stack the opposite team. Then you can add as many bots as you like using the command:
tf_bot_add blue;
You will get this for surviving 500 damage on the Haloween maps however I think this might be bugged as I unlocked it upon my death. Regardless this can be easily done with a bot Medic and either a dummy account or environmental damage