I found using ranged attacks worked really well against her. Make sure you bring a mage (use group heal every time she throws a fire ball at you, if you can't make sure you have lots of health poultices) and some archers. Bring one tank with you to attack flemeth in close range. They'll probably die, but it keeps her distracted from attacking the raged attackers for a while. Acid flasks and different bombs can be quite useful as well^^
I'm sorry I that wasn't much help... that's how I defeated her.
Having finished the game a multitude of times, I've tried all different ways with this one. It's basically an easier version of the archdemon - but judging by the question, you haven't gotten that far yet. The best way I found was to make sure you have Wynne (circle of magi 'broken circle' quest - can recruit her), or at least the 'spirit healer' specialisation for a mage. Then make sure he / she has revive, or at least group heal leveled to their character. If you keep the healer selected and out of the way, you can keep an eye on the rest of your party while they tear seven shades of shit out Flemeth, while you sit back and heal when necessary. I found this way rather quick, as well. Hope this helped! :)