I made a descent multi-sorc that does well in hell with a big time merc though.
I maxed the orb/fire ball/fire mastery and rest into the fire synergies.
She doesn't have the massive power of the pure cold or fire sorcs but with the orb not having any true synergies, you can afford the fire skills.
You have to go slow if you decide to solo the hell levels. But it can be done cuz I've been able to be very successful with this build.
You should try to get a good Nightwings, ormus, and Eschutas for the extra dam.
ZConnor