Requires a Dummy account
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 ctf_2fort; 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 Engineer on the Red Team. Then type the following command:
mp_teams_unbalance_limit 0; mp_idledealmethod 0; mp_idlemaxtime 0;
Build a Sentry, stand next to it and have your Dummy account run into its range. Once it kills him hit the G key to taunt.