Take advantage of this before they plug the hole, but you can get your TF2 achievements largely with bots.
- Start your own server, preferably with one of the handful of achievement maps.
- Bring down the console, type "sv_cheats 1".
- Introduce all the bots you want, usually just medics and scouts until you get to the specific ones that need demomen, engineers, and pyros. For all the bot commands, check out the TF2wiki.
- After all the bots are in, go "sv_cheats 0" in the console, because you can't get achievements if cheats are on. The only useful bot commands that are going to work now are bot_mimic and bot_mimic_yaw_offset.
- Type "retry" in the console, so your server now registers the sv_cheats 0 command properly.
- Achieve away! Remember, some achievements require you to die before you get them.