1) YOU ARE GOING TO BE ON YOUR OWN
As we have seen with active shooter situations, the police are minutes — sometimes many minutes — away for an "event" that is measured in seconds.
2) THE SHOOTER(S) HAVE ALREADY MADE THE DECISION TO KILL
Unlike more common street crimes, there isn't a "build-up" or "lag time" between the initial contact and the situation escalating to lethal violence. Killing is the point of the exercise, and it will begin immediately. This also means that "pre-event indicators" that serve us well in normal times are pretty much worthless here. The big pre-event indicator, as Gabe Suarez notes in the article linked above, is people in balaclavas with battle rifles.
3) THERE MAY BE MORE THAN ONE SHOOTER
And they may not all arrive in one car, or at the same time. They may have accomplices scattered throughout the city, or waiting for them at a different locations or meeting the initial aggressors at a certain time, etc. The point is, you can't know!
4) YOU CAN'T MAKE ANY ASSUMPTIONS ON WHAT THE SHOOTER(S) WILL LOOK LIKE
Nationality, age, sex, size...whatever...no assumptions whatsoever even make sense. We've already had a situation where a concealed carry holder was killer because he failed to register, or ignored, the woman behind him. Hey, Hayat Boumeddiene is cute as a button; so is a pigmy rattlesnake.
There's four more.