Might want some gloves or duct tape. Rappelling with paracord might be hard on the hands.

Your options are pretty limited. Barricade, use an improvised weapon as the shooter comes through the door, or as a last resort, go "Flight 93" on the guy.

As other posters have mentioned, this scenario is extremely rare (though there will undoubtedly be copycat incidents like after Columbine) so don't get fixated. You are still more likely to encounter a power outage or weather related survival scenario.