Ignore the gunboats! If they're convenient on the way somewhere else then take 'em out, otherwise ignore 'em.
After the Tech Center, get the bonus. I know it took me a VERY LONG time to get it. Now, its a snap. Just slow down,glide left & down to scoop it up. Where you begin your approach is critical. I used to fly to the right then make two turns down corridors to get to it, but it took too much time and often destroyed my ship. Find the best way for you to easily get it on the first try -- that's important for the time issue.
Next, the AT-AT's will not hide. Just follow the orange navigation light (always pointed up when heading in the correct direction). I like to come behind the rascals. Note there are TWO Walkers on this level.
The only thing that should divert you from the Walker is the bonus. Do not stop for anything else! When you're done with the first one, you'll head back to take out a couple Chicken Walkers (AT-STs) and Troopers. Then a Walker (AT-AT) on the way.
You can do it!
"I am the Alpha and the Omega," says the Lord God, "who is and who was and who is to come, the Almighty." (Rev. 1:8)