On normal difficulty, one tank taunting all the time (to prevent Dragon from targeting other characters) + Force Fielding the tank while your mages are blasting the Dragon, works. I've tested this tactic five times succesfully against the High Dragon. Against Flemeth, I had to do things the old fashioned way because I couldn't bring Morrigan... it's possible Flemeth is smarter than the High Dragon. I'll have to test it when I'm replaying with a PC mage.
Immunity against knockdown is recommended; also stay at the flank of the dragon all the time, to prevent the tank from being grabbed or slapped by the tail. Of course, fire resistance is important... going into battle with Drake scale armour +
would help too.