I just remember getting some mileage out of the protection glyph, if I could get Alistair to stay in it. I don't remember how early in the game I did it, but I wasn't managing my party very well at that point, so I had a bit of difficulty getting the ranged to stay at range, etc. Leliana kept switching to melee mode and running in and getting killed, IIRC.
Hold position command, remove Leliana's daggers, and turn off any personal range abilities. :P
Flemeth is definitely a tough fight if done traditionally, since she has an overwhelm-like ability, fire attacks, tail sweeps, and leg kicks. This allows her to deal out substantial damage in both AoE and to the tank.
But she also has a huge weakness--she has next to no maneuverability, and her only ranged options are fire attacks.
An all ranged party with fire resist gear/warmth balms can take her out fairly easily, as long as you are careful to actually police their range (hold position, manually move them back into range). Of course, you can't bring Morrigan, so your options are only Wynne, Leliana, and Shale for the naturally ranged-specced. Dragons are weak to cold damage, iirc, but IDK if there are any frost poisons. But you might have given Wynne Frost Weapons.