This was a well planned attack. They guy had an AR-15, a 12 gage pump, and .40 pistol. He fully armored. He used tear gas or something similar.
There are some situations you just can't prepare for.
Let's say there were 3 or 4 concealed carry folks in the audience who came to the decission to fight back. They would be acting independently, with no plan and no coordination. The best they could hope for is to draw the shooter's fire for a while and allow more of the innocents to escape through the exits. As was pointed out above, they would probably not realize the shooter was wearing armor... they would just keep banging away until their position in the crowd was revealed. Given the shooters great advantage in high-capacity long guns, and body armor, there is a high probability that the 3 or 4 defenders would end up shot.
Trying to prepare for this is like trying to prepare for a suicide bomber with C4 strapped to his chest.