I would go earth mixed with fire,works for rvr too..big bolt baseline bolt dd snare or dot whats left..will have the nearsight to pull long ranges debuff your matter dmg and 3 bolt in a pinch with fire subspec.
I think, your main damage line has to be on level, especially for BG or RvR. Miss chance will be counted on the base of calculation F(level of cast, level of enemy). So mix is never a good idea for caster. Exeption - caster like Sorc or Cabba who takes a mix skill to max of the group utility.
So, go each single skill you want - earth, fire or cold. I would take a fire spec for level - the highest damage, low power cost, quick casts.
Or if you will level in groups mostly - a cold line for the bomb, but first with lvl 11, where you will get a secound bomb and tanks are more strong as blockers and taunters.