I have a map of my own I designed a long time ago for achievement farming, along with various server config files and bot scripts you can execute to make the bots do certain things to help you get achievements.
Whenever you set sv_cheats to 1 to enable cheats so you can make engineer bots build or make the horseman spawn, after you set sv_cheats back to 0 you have to go into console and type
retry to reconnect to the server and start a new, clean session... otherwise achievements will continue to be disabled. So, while you can spawn him manually on a local server, you have to reconnect to start a clean session and once again get achievements, which will prevent you from making yourself invincible, be it by use of a negative hurt value or the buddha command or whatever
though it seemed bugged, it didnt take long to get all 4 of these achievements legitimately. youre still gonna need actual people for certain functions.
maybe ill post a link to my achievement farming kit after i finalize the installer and post in on fpsbanana so you guys can cheat all the other achievements too