Attack waves are built almost 50% of the way through the music. The section after the destroyer was pretty intense to set up, as the music has two interplaying layers. I spent a lot of time in GoldWave playing snippets to help me visualise the required attacks.
Here are a couple of screens, showing the new Bomber unit (will fire homing missiles when finished):
I also implemented a method for skipping to a point in the music, to aid testing. This had the unfortunate side effect of spawning all of the prior enemies in one huge wave. Unintended but entertaining!
Here's a video of the bug:
Based on the intensity of fighting later on, I'm going to add 'energy' pods dropped by the bombers. These will prevent energy from running out due to continuous firing (firing and being shot reduces the energy bar).
I still need to introduce an energy bomb secondary weapon. Holding down the button for a second or two charges it up and uses around 30% of the energy bar.
Lots still to do before I have a finished game, but it's getting there!