The Unachievable Achievment

You should know how to do that! :D

You should know that already but here is it again:
Set your game's properties to "-console".
Start game.
Edit keys. Choose the button for open the console (e.g. "TAB").
Begin the game. Skip intro.
Press "TAB". Now the console should be opened during your game.

Execute the command " ent_fire cmd command _u ".
Wait and get the achievment.
If you don't get the achievment immidiatly, you'll maybe have to execute the command again.