Break barrier at nest was a mission in Jak II. Ashelin Praxis and Torn called Jak through the Communicator and told him to attack the Metal Head nest while their army was attacking Haven City. With the nest vulnerable he had to kill Metal Kor which would cause the Metal Head army to collapse.
As soon as you exit the air train and move around a little you'll be attacked by mantis, new Metal Heads that jump around a lot in an attempt to confuse you. Just ignore them and shoot them with the Scatter Gun when they land close to you and attempt to hit you. The Blaster's aerial wastelander combo is also effective in killing them.
Shortly after a lone flying spider comes into the scene, thisone has a long ranged rapid fire attack where it fires a stream of bullets from left to right. They're immobile while doing so and are vulnerable from behind, but otherwise you'll want to hit them with the Vulcan Fury from a spot where they can't hit you (preferably at a distance or from the sides). After killing it find the nearby 'screw' you need to spin to cause a piece of machinery to rise and allow deeper access into the Nest.
More mantises and another flying spider will attack you as you move on, up until you eventually find Mar's gun and end this mission with a cutscene. It'll immediately follow up into "Attack the Metal Head nest".
The Precursor orbs in this area are permanently missable after entering the area where Metal Kor resides, meaning you cannot go back here so they can only be gotten this mission and the next.