Is it possible to defeat Flemeth in DA:O without any mages?

Solution 1:

Yes - a ranged party can take Flemeth without any mages, but it will require high fire resistance. See this wiki article on strategies to defeat Flemeth. I recommend getting Wades Superior Dragonbone Plate for whoever is tanking (very high fire resistance) and using other fire resistance boosting gear for the remainder of the party. Use Greater Warmth Balms for additional temporary boosts.

You could also try the ranged strategy demonstrated in this Youtube video (uses Shale, Zevran, Leliana, and the main character is a rogue). Supposedly this was done on Nightmare difficulty.

Solution 2:

In addition to magic users/ranged users, take the dog. He's immune to a lot of her attacks and tends to just stand there wailing on her.

Leliana is also a solid choice assuming her fire resistance is high.

You DEFINITELY have your work cut out for you without a mage healing though.

Solution 3:

Bring a lot oh health poultices and build a well ranged party. Leliana is your best choice, and also take the Mabari with you and Alistair. Set tactics "When character HP is less than 50%" --> "Use healing poultice" (major). You shiuld not have big problems :)