There are two darkspawn generals that we should kill before going after the archdemon. One is in the Alienage and the other is in the Market District. We need to clear the Market District first to break open the city.

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I suggest you go ahead and call up either the dwarves or the Redcliffe soldiers. There will be a surprising amount of enemies around, so having a few more distractions around will be quite nice. I've also seen Eamon's men down an ogre, so they aren't worthless either. Advance into the little neighborhood to the left and just try to rush the enemies. There should be several emissaries and a few ogres. Your warriors need to pick an emissary while your mage hopefully does something to stop the ogres.
There's not much else to do than grind though. Kill the mages and rush forward to the next batch of enemies. They should be scattered around the darkspawn general. He's actually not that bad. He's just a really tough hurlock alpha type. I'd class him as about as hard as a revenant. You just need to hang on and grind him down with your tank and warrior. You should also have some other warriors with you to help, so hopefully it won't be too bad.
Make sure that you check his body. It should have the helm, Corruption, on it. This helmet offers an absurd amount of defense against spirit magic. This is the main attack of the archdemon, so you'll probably want to pick it up along with some good health potions on his body. You can advance out through the exit to the alienage for our next stop.