Shale, Wynne, and Morrigan is typically the best set-up you're going to get for higher difficulties, especially if your main is also a mage. Shale takes hits like a champ and can get the attention of all your enemies. Tossing Force Field down to protect Shale if her HP gets too low can save your ass. Otherwise, Wynne keeps Shale alive while you and Morrigan pummel the hell out of everything else from a distance.