Well this is the list I kept from the last time I played an evil conjurer. This is how I'd divide the spells into categories. I'm sure this is based on personal playstyle, but it may help with the AI.
Category 1- Aura, Barrier, Shield, Protective Shield, Fly

Category 2- Acid Storm, Hellfire, Acid Blast, Unholy Word, Harm, Vampiric touch, Ice needles, fireball

Category 3- Poison, Plague, Blindness, Weaken, Fumble, Fear

Category 4- Slow, Wither, Rot, Confuse, Soften, Curse

I'd love it if devils did all of cat 1 out of combat. Then do a cat 4, cat 3, cat 2 rotation or possibly a cat 4, cat 3, cat 3, cat 2 rotation. That would go a long way with devils. One other thing about devils is that they quit using their spells when attacked. I can understand archons doing this, but perhaps devils should still spell up whoever is striking them.

On a sidenot is that nightgaunt echo possible?