Survive the ADS Cannon with over 50% shield strength remaining.

As soon as you cross the big cap where the asteroids are coming at you. Have the door close and SAVE right before you get on the gun. If you don't save and go below 50% shield strength on the gun you will not get the achievement. I just got this achievement with 77 shield remaining. What you need to do is focus on the big rocks and the really fast ones. Fire both guns at the same time. It will kill the big rocks a lot faster. Remember that the sequence is scripted so it will be different every time. If it goes to 49 quit out and load again you will be right next to the gun for another try.

It's also a Star Wars reference. :)