If I could tell he was wearing body armor the best choice, as hard as it may be, would be to lay in wait behind cover, when he got close, pop up and shoot up into his head. Hitting a small moving target, like a head, in the chaos of a crowded theater at any kind of range, would be very close to impossible, would certainly turn his attention towards you and likely get you killed.
If the original shot would be tough, think how hard follow up shots would be while dodging his now incoming rounds directed at you.
Another option would be to stay behind cover until his attention is away from you or he's reloading, whatever, and move. Close the gap and take him out. Risky. Need to pull on the big boy pants, but, if you are going to do something do something hard and follow through.
Both of those options also greatly minimize the threat to the innocents.
We do a two full day active shooter training every two years. Everything about taking on an active shooter sucks. Everything.