This Achievement is a bit of a pain in the butt. I found that out of the 3 curses, this was the least chosen one.
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;
Select Spy as your class. There are 3 curses, Big heads, Underwater and No Guns. When they say no guns, go behind each bot and stab them in the back. The Curse will spawn all the bots in front of you. Once you are done, you can do other achievements while you wait or just sit there and wait for the next curse.