While grinding this difficulty, I always thought it was really slow to just sit and let the final Transport just fly away slowly. So after destroying the second one, I go down and throw all my grenades and just go to town on it. But you need to be quick to get back up again and into position for the boss fight, or you might screw up the pattern. It's obviously far from perfect, but at least I beat Dandelyon1830's time, so I'm happy!