To easily become a master in illusion, create a charm spell in the arcane university that is lowest magnitude and lasts for one second. Then find someone who stands still for a long time, like a shopkeeper. Draw your sword, hold down block, and keep pressing R2 to cast the spell. The purpose of holding down block with the weapon drawn is it makes the spell cast faster than normally! So, I got level 100 illusion in no time at all, and I can make myself 100% chameleon for 40 seconds. This helps a lot with the Thieves Guild and the Dark Brotherhood quests. Enjoy!