Trigger_hurt with negative damage value. A trigger_multiple that triggers a multimanager, wich triggers the trigger_hurt on, and after a little while off. A multimanager can trigger the same entity several times, with a certain delay, so that last trigger_once of you is quite useless since the multimanager can do it too.
The whole door isn't necessary here (since a door gives damage when it crushes you, that can also be used instead of the trigger_hurt. However, the player needs to be crushed between something and the door. A trigger_hurt only would be much better and easier to work with)
Now the only thing I don't know, since I'm not into CS mapping specific, how to reset a trigger_multiple after a round. Maybe it happens automatically? I don't know.