iirc that's the final battle of the corporation path. You can't flee, and after a while the enemy will hack the Starhammer from your control and bring it to the north portal - where you will lose if it reaches there and escapes.

When the mission starts, proceed normally to the rendezvous point in the middle of the map. Once the shooting starts, immediately head southwards near the portal there. The Starhammer should follow you. All this while you'll be getting shot at and reinforcements will keep drifting from the corporation bases. If you pace yourself well though, you should have a little breathing room in between the assault waves because the enemy will normally head towards your last position. At this point try not to use up all your skills. Particularly, save any shield polarisation skills you have.

By the time you reach the bottom of the map you'll get a short scene where another enemy appears (not gonna spoil it for you unless you want me to). You'll soon lose control of the Starhammer, so trace your steps back northwards to the middle of the map. With luck you'll engage the separate wings of enemies there separately instead of all in one go. Once the first 2-4 waves have been disposed of the Starhammer will probably reach the middle of the map already so you've only got a little time left. Fortunately there should only be 1-2 more enemy wings plus the main bad guy. Throw everything including your brother and the kitchen sink at it. Win. Gloat, head towards a portal to exit. Savour the cutscene.