An easier path, especially for higher-level folks, is to enter Qeynos Harbor (harbor guards are around level 35), either via the sewer system path above, or from Graystone Yard (guards are around level 15). It is possible to scale the walls of the Hall of Steel to gain access to the secret back door into South Qeynos behind the building. This entrance to South Qeynos places you on the waterfront by the Concordium Tower, and all you must do is follow the wall to your right from here, there will be no guards.
Make your way past the back side of the Concordium Tower, avoiding the guards. There you will find a cellar door ( 760, -20, 102 ) /waypoint 760, -20, 102 opening to The Summoning Chamber behind a tree and against the wall.
Invis and go back out the door, and sneak up the stairs.
Enter the room at the top of the stairs and go to the corner to your left. You only need to kill the Concordium summoner, who may be body pulled by advancing to the bench behind her. Pull her back to the corner and kill her.
After you kill the Concordium summoner, The Portal Beast will come through the summoning portal. Kill it. This creature may also be body pulled into the corner and killed without aggroing the other Concordium personnel in the room.