That makes the most sense, to balance out the build.

If I am doing all this cold damage, I might as well have a steady source of fire damage. I went back to the rogue camp and grabbed Floria who uses fire arrows. She also seems to be leveling up real quick in the sewers.

Edit:

Came across a huge cold immune in Nightmare and the fire merc saved the day. I was summoming constantly to buy her the time to deal damage. Went through a lot of mana and summons, but the team work was perfect.

I was reading that the normal difficulty mercs have better stats. I am guessing this is only minor or at least in my version. The first merc I leveled up in nightmare did 42-172 compared to 45-175. Then I tried a couple more mercs and it took them two extra levels to do equal damage.

I don't want to go back to the normal difficulty for a merc, so I will just keep the new one from nightmare.