Fire mercenaries just need to use Meteor more instead of Fireball in Hell | PD2 season 4
To make it easy to see that the Fireball wasn't useful in Hell, I had to give the Fire mercenary the best gear to get him max block. Normally he wouldn't have survived much longer.
In Hell, there are too many Extra Fast and Immune Fire monsters, which makes it difficult for the Fire mercenary if he has low quality gear. Unlike in Normal and Nightmare, Extra Fast monsters in Hell are virtually immune to Fireball's "short stun" effects even though they have no immune fire. So I think it's better to change the AI of the fire mer in Hell, allowing him to cast more Meteors in Hell.
This would balance out the Cold mercenary being able to slow or freeze monsters with his spell, or the Lightning mercenary being able to destroy Immune Lighting with Static Field and shock the monsters.