Given an active shooter threats, what is the solution if not a good guy with a gun?

Why are you assuming active shooter threats need to exist to begin with? There’s only one country it regularly happens in.

More gun homicides per capita happen in states with looser gun laws by and large. The good guy with a gun is largely a myth as compared to “i have a gun and I’m going to use it to kill you”.

The existence of active shooter threats and especially their prevalence shows that the best protection - prevention - isn't working. And in the US, the best possible protection (strict gun ownership laws) is prevented based on an ideology that arguably misreads the Constitution on purpose to keep a whole industry alive.

And should it come to an actual active shooter, the response needs to be a properly equipped and trained police force acting against it rather than sitting it out outside the building. That is not the proverbial "good guy with a gun" because that's implied to be a random untrained civilian on the scene.

Where’s the data supporting that that’s a good solution and prevents casualties in the first place

Ever see Takeshi's Castle?

This is not super on topic, but ANW is the US version of the Japanese TV show Sasuke, which is itself inspired by the Adventure Zone segment on Takeshi's Castle. So, yes, it is connected.