I’ve improved the first level boss by adding a top saucer to it. Now it has a top saucer section.
To destroy it, you have it shoot the middle support section, which has 4 parts (or legs), as you hit each one they fall down, and once all four have been hit and have collapsed the top saucer section falls to the ground and crashed. What remains of the boss then turns to the side (as mentioned before) and the final part of the boss battle commences.
Here’s a GIF I took of the boss battle, but I’ve since added movement to the boss, now it moves from side to side.
That more or less makes the first level complete. There are around 150 enemies and buildings to destroy. I still need to add things like effects and explosions, but I’m relieved to have finished the first level.