Funny thing... I posted that last night while drinking and mapping... Thinking back on it this morning, something dawned on me: I was the only one on the map.
For all I know it may have only been damaging me as I was the one triggering it.
Regardless I decided to err on the side of safe and modify the design a little.
I just dropped a aaatrigger brush over the entire level and set it as a trigger_hurt. Set up the damage it deals and set it's target to a trigger relay that responds back to it with the off switch.
The rest is pretty much the same. But you need to add a relay.
Set up your switch and target your relay that sends the on switch to the trigger_hurt. Now the trigger_hurt will damage everyone inside it and fire the relay which turns it back off.
I have no idea if what I said last night actually works or not as I've elected to modify my map before the rediculously long compile time it's going to have, not after.