General Mercenary Advice
The primary aim in hiring mercenaries is to cover for the gaps in a flaming druid's skill set. The most glaring of these is that the bulk of the damage you will be doing is fire elemental damage. This is fine for the majority of the game, but on "Hell" and "Nightmare" difficulty in the later acts you will start coming across monsters that are immune to fire damage. Distracting these enemies while you hit them with your weaker nonfire attacks or just killing them outright will be the main task of any mercenary you hire.
Desert Mercenary
The spear-wielding desert mercenaries of Act II use beneficial auras that affect not only themselves but your character and any other hirelings or summons you have with you as well. On normal and hell difficulties, the defensive mercenary with his defiance aura is your best bet, but the best desert mercenaries are found once you reach nightmare difficulty. The defensive hireling will then use the freeze aura, which will slow all enemies and freeze some completely, adding cold damage and allowing you to shatter them.
Iron Wolves
The Iron Wolves are available in Act III and cast offensive spells at your enemies. Choose the ice iron wolf. At higher levels he will be able to take care of fire-immune enemies (which often are vulnerable to ice attacks) with his ice balls by himself. Even against enemies that are not particularly vulnerable to ice, his ice balls will freeze them solid in their tracks for a short time, providing helpful crowd control.
Barbarians
The barbarians of Act V are not as popular a choice as the desert mercenaries or the iron wolves for fissure druids, but can be useful in certain situations. They fight more or less the same way as the desert mercenaries, though they do not get the benefit of an aura. What they do have, however, is a lot of health. Barbarians are the best choice if you need to keep a group of monsters distracted long enough to take them out from afar as they can take massive amounts of punishment before going down.