most illusion becomes useless once at a certain level
Fury initially affects targets of level 6 or lower (Novice)
Calm and Fear initially affects targets of level 9 or lower (Apprentice)
Frenzy initially affects targets of level 14 or lower (Adept)
Animage adds +8 to the maximum level affected (vs animals, all spells)
Kindred Mage adds +10 to the maximum level affected (vs people, all spells)
Hypnotic Gaze adds +8 to the maximum level affected (calm spells, all targets)
Aspect of Terror adds +10 to the maximum level affected (fear spells, all targets)
Rage adds +10 to the maximum level affected (frenzy spells, all targets)
Illusion Dual Casting approximately (more than) doubles the maximum level affected (all spells and targets)
With these perks, the above mentioned spells will now affect:
Vs Animals:
Fury: (6+8+10)*2 = 48 (Frenzy replaces)
Calm: (9+8+8)*2 = 50
Fear: (9+8+10)*2 = 54
Frenzy: (14+8+10)*2 = 64
Vs People:
Fury: (6+10+10)*2 = 52
Calm: (9+10+8)*2 = 54
Fear: (9+10+10)*2 = 58
Frenzy:(14+10+10)*2 = 68
Vs Undead, Daedra, and Automatons:
Fury: (6+10)*2 = 32 (Frenzy replaces)
Calm: (9+8)*2 = 34
Fear: (9+10)*2 = 38
Frenzy:(14+10)*2 = 48
These values are actually less than the actual amount since Illusion Dual Casting slightly more than doubles the potency of illusion spells. Enemies stop scaling past level 50, so as long as these spells are capable of affecting an enemy of at least level 50, then they will work on all enemies.
http://www.uesp.net/wiki/Skyrim:Illusion