Playing as a pure illusionist is now viable, as nothing is totally immune to the effects of illusion spells.
Low-level undead are particularly easy targets.
Automatons are vulnerable, however they will be more difficult to control.
Even formidable enemies such as dragon priests, ebony vampires, and dragons can be killed via Illusion. However, this will be no easy feat as they have extremely high resistances that you will need to train and have the right gear to overcome.