We don't have access to modding the AI to that level.

The most fun I ever had was with Age of Empires II where you could make your own AI file rulesets. Some of those developed by users were amazingly hard to beat and sometimes you lost. Given that all of my games so far of FE resulted in me leading the 2nd place AI by a 2-1 margin makes me think the game would benefit greatly if access was enabled. We can only hope.

You should be able to do what you want.

Make the AI values for those spells work for you. The highest valued spell should make the spider use it first and then go through the others in succession.

Only problem with raledon's suggestion would be that if they are in a situation where they cannot be knocked back, they would still utilize that ability instead of attacking.   There is definitely code to run away, for some reason *poisoned* units often do.   I suspect that might be a bad tag in the AI code, and the PoisonER is supposed to run away after poisoning.   

You could however give them a defensive ability, and have them utilize that as a priority over attacking.