I have actually beaten Flemmeth and the High Dragon since. I was around level 19 when I beat both. I beat both of them with only my main character left alive. I used Alastair, myself, Wynne and Leliana for Flemmeth and Alastair, myslef, Wynne and Morrigan for the High Dragon. And I only had about 20 health polutices. It's not too bad if you have Wynne healing and Morrigan attacking and the Dragons focus on killing Alastair lol.
I used Shale, Wynne, and Oghren, all about level 12-13, for Flemmeth. Wynne was the healer, Shale the tank, with Oghren and myself as damage. After Shale went down, I went down, and I followed up with Oghren for the win.