No combo using fire blast is ever going to be unique or even rare, so if that's your only reason to consider that option, put your mind at ease and go trap/son. 
I dozed through the Praetorian AVs on my Traps / Dark, a combination that is not optimal for AVs. IIRC, this toon can drop two pylons in something like 4:30 (it's been a while).
Just IME and all that, but Traps has enough -regen, -resistance, +defense, -damage, and places for damage procs (and ability to set off Reactive at the Incarnate level) that your secondary choice is largely irrelevant for soloing AVs. Some combinations will be faster, but none will be incapable of soloing AVs.
Really, traps/, rad/, and cold/ anything will do fine against AVs. But /fire will not be unique in any way honestly. Everyone and their mother has a boatload of fire characters.
I really cant make up my mind I want a Trap/Fire for uniqueness but im not sure if it can be done or a Trap/Sonic for damage and debuff reasons can someone please help me decide