In the area where you see the first Titan, after it kills the soldiers and 
before it shoots
the door open, throw a Backlash Grenade to deflect the shot. The door will 
remain
unopened; however the Titan will run towards the door but cannot get 
through. You
can still kill it, however the door will never open and you will have to 
quit to the main
menu and continue from where you last saved.