Once the terrorist has burst onto the flightdeck, there is probably precious little anyone can do.
Of course now we have (or are just about to have in our company) locked, armoured doors the theory is that no one will be able to burst onto the flightdeck. That's the theory, in fact the door will still be opened from time to time in the air and it will still be possible for someone to force their way onto the flightdeck during that time. The BIG difference now is that nobody can then get him out again, all he needs to do is overpower whoever is left on the flightdeck and then he can relax, take his time, and select his target in peace while he listens to people trying in vain to break (and now shoot) the door down. Scary stuff.
In my opinion we are better off with a curtain across the flightdeck than a single armoured door. The only solution is the (very expensive, hence not going to happen) El Al solution of a double door system. This ensures that there is never an open path from the cabin to the cockpit.