I agree about the difference in training levels, but there's a lot of killing possible between (attack foiled, no friendlies dead) and what went down there. (I was surprised at how long it took to kill so few. Does anyone know if the area was old style construction with lots of stone and mud in the walls? Lots of rebuilding and redesigning to create a maze?)
The people carrying in "may issue" areas like Los Angeles are like the Mumbai police, politically connected drones, if not criminals. They would be nearly useless for defense.
The LA police? "When seconds count..." A group of terrorists could secure a mall (or other area with similar pedestrian traffic choke points such as a school, church, hotel), assign some to prevent entry, some to prevent exit, and some to hunt and kill. They could have control before the police had time to react. So what if all of them were to die? They are there to become martyrs.
Strategy? The bulk of the forces are in vans or buses in the parking lot. Scouts to find the security staff and any stray cops. Signal the blocking forces to get into position to seize the entries. (No indication of trouble to set off alarms up to this point.)
Signal the attack.
The scouts kill the obvious resistance, the blocking forces take the doors, the main forces dismount and charge in with weapons, vests, and hooligan bars. Total time to effectively seal the facility so the massacre can begin, less than one minute. With a bit of training and proper target selection and timing, they could have a thousand dead before the SWAT team got their pants on.
Add a couple of sniper nests with silenced rifles outside the facility to buy extra time for the executioners inside and to increase the death rate among the responders. Add a couple of car bombs to the parking lot.