This is the last stretch of the game. Buy everything you'll need for this last fight. The recommended items for the final jaunt include the Plasma Cutter, Ripper, Contact Beam, and Line Gun. Proceed through the next area and run like mad. There are swarms of enemies, including invincible Necromorphs. Save time by killing the minimal amount and dash to the exit.
Take the elevator up to the above floor and smash the fuse to the last showdown. Save one last time and get ready for the end. You'll face Tiedemann in a quick time event. It's very sudden, so be ready to pound the on screen commands.
After laying waste to the military leader, it's just you and the Marker left. Your absolute top priority during this battle is to kill Nicole each time she spawns. If she manages to get too close she can instantly kill you in one strike. The other Necromorphs are less of a problem, but still continue to hit them with your Line Gun and Ripper.
After so much torment, the Marker will eventually unveil itself. Use this time to hit it with your Contact Beam and Plasma Cutter. Nicole will reappear after doing enough damage to the Marker. Once again kill her off quickly. Repeat this process until the Marker is finally destroyed.
After a short bit of credits, you'll be swooped away by Ellie. Navigate passed the flying debris and complete one last quick time event to win Dead Space 2.