For my Impossible Scenarios map, I'd like Raven to actually cast spells when they are attacked. Same for High Templars, Ghosts, and such.

How do I create such a customized AI?

As far as I know, for a custom AI you'd have to set up triggers for each unit based on what you'd want them to do. For example with a High Templar, you can set it up so that they only cast Psi Storm when they are near a group of units as opposed to one unit. This is just my guess, though, as I had to do this for custom units I've made.

I ended up doing triggers, just like you said, using the "Order Unit" action and the "Unit is attacked" event. Works well.

Sweet, good to know. It gets tricky sometimes with custom abilities, but other than that I like how simple it is to set up custom AI.