Your default solution should be to be invisable during, and witness afterwards. Unless and until the situation turns specifically deadly.

I will present the SOPs that were used at a liquor store back in the old neighborhood that had frequent robberies.

All the employees were encouraged to be armed, not just the specific register guy. As long as the robber was mostly coherent, hand over the money in the register, and let him go. They practiced good cash control, made frequent drops into drop safe, and would only have a few hundred in the register at any time.

But IF :
1. The robber was noticeably high and unpredictable.
2. Robber(s) would endevor to herd people into back room.
3. Robber initiated an attack.

Then everyone would go into OK Corral mode.

One occasion my friend wasn't carrying his usual 2.5in Python when a robber came over the counter at him. He used his Gerber Mk I knife to cut threw the gun arm down to the bone.