Go for it!
The right Merc can make up for physical immune/cold immune.
I'm level 38 in Nightmare Act 2.
I was messing around and changed mercs to see how a new one would be. I grabbed a level 35 defensive merc but I can't tell what the aura is. I was trying to get one with might to improve the damage of my party.
This merc is 3 levels lower so he does seem to take a lot more damage. I also think it's because he gets too close to the battles. I just remembered I didn't equip him either, so that's a big factor.
I don't know, I am comfortable with the rogue because I think she can stay alive better. Still wondering if I should change over to one that uses fire arrows or keep mine equiped with a different weapon. <img src="/ubbthreads/images/graemlins/rolleyes.gif" alt="" />
Do you know if there are more monsters immuned to cold or fire?