There are several Boss-level guards and NPC's in this area. Your ogre that you recruited in the last main quest will be invaluable here, as its Hurl ability can do heavy damage to any opponent, and it can break down most gates. Making use of the shriek's stealth ability is also a good tactic.
Enthrall the shriek. You may need to execute one of your existing thralls to do it, but do not execute the ogre. Besides being helpful in battle, you'll need it in order to complete the Ogre's Keeper achievement.
Have your other party members hold position near the gate and stealth the Shriek. Use it to kill the nearby guards before they have a chance to use the Ballistae on the rest of your party members. Once the guards are engaged with the shriek, you can allow the rest of your party to move freely and help finish the guards off, or you can just wait until they are all dead before letting them move freely (the shriek is powerful enough to take them on its own without taking too much damage).
Kill Bann Teagan, the human commander operating out of the Arl of Redcliffe's Estate, in the small courtyard in the southwest part of the district. He will have several low-level guards with him, which are easy to pick off. Direct the ogre to focus solely on Teagan and let the other thralls deal with the guards.
You now need to kill 10 civilians. The unarmed civilians will respawn randomly throughout the district, along with multiple guards and named NPC's. Only the unarmed ones count towards completion of this quest.
Lead the genlock sapper to the Alienage gate. The sapper will be in the same courtyard where you fought Bann Teagan. Approach him and he will automatically follow you. When you get close enough to the gates to the Alienage he will start setting up the explosives (you don't need to click on the gate or give him any other commands).
As soon as the gates have been destroyed, you will receive a warning from the archdemon the guard is rearming. Run around Wade's Emporium to find Herren and Master Wade standing near many knights--these warriors are well equipped so bring your forces. Cornering the couple against the Chantry wall reveals some interesting facts about Herren, but otherwise is not necessary for your mission. Click on the cache of weapons to get the Sapper destroy them.
Rewards EditForge Master's Hammer
Red Steel (Tier 5)
Requires: 32 strength
Critical chance: 0.70%
Armor penetration: 11.20
Strength modifier: 1.25
+25% fire resistance
+6 attack - Found on Oghren. Redcliffe Shield
Varies (Tier 1-7)
Requires: 18 strength
Strength modifier: 1.00 - Found on Bann Teagan. Spirit Band
This ring feels as light as air.
-20% fatigue - Found on Wynne.