You can beat all difficulties with all classes, they all have the same total base stats, they just use different abilities and it all depends on how you use them.
Personally I favor heal-bot hydro, electric-shadow psycho and speed bio.
I'd have to say the best classes are a Psychological (wraith build) and a biological (speed build), Ihave one of each at level 30, both are pretty brutal and hammer for ALLOT of damage consistently, I think the wraith just edges out the speed bio though, why you ask? well both have skill that raises speed by 400% but the wraith form lasts 5 turns while the shadow blend ability from the bio only lasts 2.