Which he did after learning to control the engery in the twisting nether after studying it while the other titans didnt, before that if he was attacked with fel magic on the scale he wielded against the other titans he would of been just as susceptible to it

Aggramar proved a quick study and soon earned Sargeras' admiration, and became the warrior's trusted lieutenant. The two fought shoulder to shoulder against the endless demons of the Nether. This allowed Sargeras to more closely study the nature of the Nether, and by learning how to manipulate the chaotic realm's magical energies, he created a prison world known as Mardum to contain the demons he had defeated