There is rarely a bad time to brainstorm, so here’s some ideas.
We don’t have the battle system mechanics fleshed out yet, but I’ll be making some assumptions of those here and there - nothing that we can’t change later if it turns out not to be fun/make sense.
The Illusionist, not being a library rat like the more scholastic Wizards, or secluded as a Necromancer would be, would probably be reasonably hardy and healthy in comparison to other practicers of magic in general.
Most magic users would have magic that’s “fire and forget”, that, after cast (some pre-casting time possibly ensuing), immediately affects the targets. The Illusionist, on the other hand, while faster to act, could probably have to keep his focus on his target(s - perhaps at least two are possible) so their effects stay active. Therefore, most of their abilities would be continuous in nature.
On the other hand, if an illusionist ability, by its nature, targets several enemies, (like sleep-inducing Hypnosis, if it’s anything like the skill as used by RL top-hat magicians) then that limit may well be broken.
This targeting might get a bit hard to track down (I hope not 
), but on the other hand, we could make them not limited by MP, only by their ability to stay concentrated on their targets.
Presumably, suffering particularly strong physical or mental blows would break his concentration, so it would be wise to have a “tank” around to protect them.
They have skill that mess up enemies’ senses, making them see what is not in the battlefield, or hide things/people that should be in plain view.
Other techniques are related to making others feel what they shouldn’t (such as pain, or damage), and vice versa.
As for skills, some off the top of my head:
Hypnosis:
The Illusionist tries to keep several targets at bay in the general direction he’s facing, by inducing them to sleep; Possibly with an intermediate period of sleepyness before the full effect kicks in.
Ethereal(?) Immolation:
The target feels like he’s in flames, possibly affecting his behavior, but mostly acting as a multiplier to physical damage. Underwater enemies would probably be immune to this, as they wouldn’t believe to be burning.
As a variation, if the negative damage resistance seems unlikely to be caused by fire, we can call it Ethereal(?) Claw, the target feeling like he’s being cut to shreds, his open wounds making him more vulnerable to physical blows.
Invisibility:
The illusionist hides one of the targets from the enemies; if any of them don’t use their eyesight as the main way to detect other entities, then the Illusionist either takes care of that, applying the illusions needed (automatically, so it won’t make a difference to the player); That, or he just doesn’t, and the illusion is lost on bats and wolves, etc.
Cacophony:
The target(s) hears multitudes of screaming, howling voices, hampering their concentration, ability to cast spells, etc.
Illusory Bind:
The targets feel like they’re constricted by heavy chains, making them immobile, unable to attack physically, and possibly causing them some damage over time. Fire-elemental (?) enemies, that would presumably melt away the chains, were they real, would be immune.
It’s getting late, so if I think of anything else, I’ll be quick to edit this.