Death Star Trench:
If you turn the detail to its lowest setting, most of the difficult obstacles will not appear. For example with the detail on maximum there are several yellow girders that stretch across the width of the trench which you have to avoid. With the detail down most of these will be gone, making the mission much easier.

Weapons, Invincibility, Walk Through Walls:
During the mission press the Esc button. Use the bar at the to of the screen. That will move you to the next screen. At the bottom of the screen there will be three bars in a group click all three bars one time. Then go back to the mission. You will be able to pass through objects, be unkillable, and have unlimited weapons. On the trench run if you run into the walls or the ground you will die