I have been duo'ing on nightmare for a while and im just doing everything now before i finish the game again here are my thoughts.
You will need multiple staffs, possibly one of each type and they will need to be quality. You cant afford to come up against a foe who is resistant to your damage.
One of the hardest fights i have had so far is against ishmael as he changes form one is immune to fire damage the other lightning makes it really hard if you have a fire damage staff equipped.
The hardest foes are immune to fire it becomes just a battle of attrition, as most of our high damage spells are from the fire tree and on the opposite end the easiest are the one vulnerable to fire.
When the rift tree opens up i would take upgraded veilstrike and mana regen and the damage reduction i wouldnt get it all( at least straight away anyway)
Offensive is the best defense in alot of situations, you cant just absorb all the damage you take it will leak through your defences, the quicker you can take out opponents the easier it will be.
Frost is really lack luster damage wise.
I dont think awesome gear is as useful for us as it is for other classes, especially rogues.
Im sorry im tired ill post more if that helps and what i have said makes sense.