Hint: Defeating the AT-STs
To quickly defeat the AT-STs in level 3, duck until one approaches. When it is directly overhead, swing your lightsaber upward. Then jump over the top of it and swing it again. Your jump becomes charged when ducking.
Hint: Defeating the AT-AT Walkers
The Imperial AT-AT Walkers in level 2 are defeated easier on foot with Luke. Getting between its legs and press Up + A.