You're welcome :) .

I personally would use the frozen (lowest-level) armour, but that's just me.

Fire wall can be incredibly devastating with enough points in that and the mastery. Thousands of damage per second. Of course, then you probably won't want to hit people with your sword, because the fire wall dwarves the damage you'd do that way :p .

Also, if you only deal fire damage, you'll have problems against physical and fire immune monsters.

Charged bolt is wonderfully devastating, but it needs a significant investment in both it and lightning mastery. *shrugs*.

Also, I read that currently (possibly a bug to be fixed in 1.10 :/ ), elemental damage from weapons is increased by the masteries. So if you have some lightning damage and a high-level mastery, this would help you deal with the dual-immunes.

As merc, you'd probably either want a might aura (for the added damage and the ability to deal a lot of physical himself) or perhaps an Iron Wolf to complement your own skills.

*shrugs*. I don't know :p . I'm tired and these are mostly random ponderings :p ...