Be mindful that this place would be inaccessible after your battle with Gadolt in Agniratha far later on. Also, don't forget to buy anti-Mechon weapons or your team can't dent the Mechons at all. After talking Dickson and leaving Dickson and Alvis after the cutscene, continue on your journey to the far north left where the next battle would happen. Make sure you seriously finish any surprise quests before you leave this area.
Mumkhar last stand[edit]
Port Access Pathway is the area you should be reaching. Once you get to that place a major battle when Mumkhar brings his armored troops to take care of you guys. Remember to break, topple and daze the oppponents.
Perhaps as a surprise cutscene, Shulk stops Dunban from killing Mumkhar. Since Shulk realises after the knowing that the Mechon has green light that allows Bionis tissue to disappear, he strongly wants to know why Egil is putting people into machines. In the end, Mumkhar has sealed his owned death by his attack.
If there's nothing else, make your way to the entrance of Galahad Fortress through the remaining areas of Sword Valley. It should be at a bottom area then going about north on the map until you see a doorway near the ether river.