“Seek out the source of the Sunbreaker's signal.”
Head up the stairs found the N/Gen Branch. Go into the building that sits above the area. Inside it are more Vex waiting for you.
Use the doorway as cover. Duck in and out of it and pick off the various Goblins and Harpies that are firnig at you. Once you can clear a path, then jump up onto the ledge in front of you. From there, turn to the right and head through up the stairs there into the next room. A pair of Goblins are waiting at the top though. Take them down quickly and continue forward.
Just past them is a pair of Harpies that you need to deal with. Just go forward through the room with the small pond. Start down the stairs just after that. Be on guard as you descend though. This leads you into Dig Site 4. jUmp over the railing or go down the stairs to the level below you. Go through the corridor on the left.
There is a pair of Goblins in rest mode just in front of you. Destroy them and head through to the next room on the right. There is not anything in here so go forward and through the door on the left past the stone in the middle of the room.
On the landing below you is another pair of Goblins. However, there are a number more Vex in the room just in front of them. Take out the Goblins quickly head either left or right. Either way you will need to deal with a Hobgoblin and another Goblin. The Goblin is toward the middle and the Hobgoblin is on the right. Deal with them or run out to the left to avoid them. You have a big fight coming so take a moment to reload everything. Pick off the few Goblins that are around the area.