Complete all other quests before giving the ingredients to the alchemist and having him make the cure.
If you run past the fire elementals, there are 4 strange candles; for each one extinguished, a fire elemental will die, though you do not get the experience points that way.
A better tactic would be to have one character use a ranged attack (an archer set to aggressive works well) while the other hit and runs so as to keep the elementals' attention, but avoids being surrounded. Two melee characters would be a little tougher. If you attack, then withdraw, they should hopefully spread out a bit coming after you, allowing you to take them on 1 or 2 at a time.
