From my experience of creating AI for my AOS map, all I need to do is turn on AI for the AI players, and make sure the units actually HAVE the spells and that they are leveled up (you could do this by triggers, or, if the unit is pre-placed on the map double click on it on the WE and set it to the spell's highest level, etc.). This works for autocasting spells too, tick the box that says "Autocast".
Then let her rip. Try it.
What I am not entirely sure of is the tab in Object Editor on the spell that says "Order String". I've been wondering if this is what you want the AI units to think of the spell or if it's something else. For instance if you created a custom spell based on Channel which traps units like one of those Lasso spells, maybe you should change the Order String to Lasso instead of leaving it blank so the units using it will treat it as a lasso spell and actually lasso people with it. But I could be wrong on this one.