Just use all of them as you go. You'll obviously be using Illusion to start your fights (Use Rage to get them fighting each other), Destruction to finish the enemies off, and Restoration to heal and block magic. Remember, Wards block dragon fire, so dragon fights are a great opportunity to level Restoration up.
Some other advice would be to keep staves handy so you can save on mana during heavy battles. Also be sure to recruit a melee fighter to tank for you. (I liked Janassa)
i have almost no experience with illusion magic, but yeah just level them as you play. experiment with the playstyles.
if for example restoration is falling behind a bit, maybe allow yourself to purposely get hurt a bit more than normal so you can heal more. or if destruction is falling behind focus more on that. but just use them naturally and how you like and it'll even itself out.
The "influential" spells in Illusion are invaluable. Just like wereninjah stated—use them the start fights, whittle down the enemy count before you even jump in the battle, then calm some to take your targets one at a time.