How do you solve that problem by going fireball? Your merc will then have to face fire immunes and there are a lot of them in Hell A1 (just think of all types of Fallens and their Shamans).
So
#1: you teleport around immunes anyway. Then you can choose whatever element you like, fireball or blizzard with all synergies
or
#2: hybridize your build. Fireball + Blizzard gives a good option to clear everything while being fairly safe. I use this combo as a ladder-restart build, because you can solve almost every problem without specific gear. I always add in static field, but that might not be a good idea in Player 8 HC

most popular hybrid options are meteor+orb, hydra+orb and firewall+orb
my personal favorite hybrid builld is hydra+orb as there's very few areas in the game that you cannot smash through

oh yeah, I did not think about the hydra have not used that spell in a year (literally), but this might be a very good call for HC!
although I personally favor blizzard over orb, but that is a preference choice I guess