This line of movement should feature one repeating large asteroid that comes in at a slope of 2. The completely drawn in figure should be its relative location once the Danio has come in far in close enough for the full sprite to be visible. This applies to the other moving Large Asteroids pictured as well. The second asteroid having a slope of -4 and the other of 4. All three come in from the top and fly towards the bottom. Speed to be experimented with, but nothing speedy since we can only move up and down and this is the first level. Note that once the asteroid has disappeared down the bottom, another should, after a short delay, take its place along that line of movement.
This Small UFO sweeps in when his entrance square is roughly 160 pixels away from the right side of the screen, moving left to grab up the minerals and then swooping off.
This Small UFO sweeps in when his entrance square is roughly 160 pixels away from the left side of the screen, moving right to grab up the minerals and then swooping off.
There is another screen's worth (ie 640x480) of background featuring maybe a stray small UFO swooping in from the bottom and out through the top quickly or a small, lone medium asteroid, but otherwise empty for a full 640 pixels, which then cues the boss encounter.
The UFO enters from the right side of the screen, vertically in the middle. It moves out so that its right edge is 128 pixels out from the right side of the screen.
It drops its laser weapon down.
It charges for a long while, menacingly.
A small, dinky, slow laser double fires off, slowly moving by to be easily dodged by the player.
The UFO turns slightly red, shaking about angrily.
It charges up again, quickly, moving upwards and firing off shots as seen in the (as-of-yet not yet) uploaded outlay of the boss fight. I'll throw some details on the angry shaking movement mentioned in step 5 as well.
Etc, turning deeper red and firing off more shots til it eventually overheats and blows up. Yay.
Pulse weapon that had dropped down in step 2 remains, floating towards the player and remaining in reach and not ending the level til the player picks it up.