Alternatively, you could also use Leliana for additional ranged damage instead of Morrigan. I was assassin rogue main as well, with Wynne for healing, and I believe I had Morrigan for cold damage, which the dragon is weaker to. Get her equipped with some fire resistance crystals (Large Brilliant are the best) and she's quite the powerhouse, especially with Stoneheart as a skill. I would highly recommend using her because of that. I ended up beating the High Dragon with Shale as a main tank because she is immune to the Dragon Bite of Doom (not the official name!), which is a one-hit kill for any other character.
