Glad you enjoyed it! Thanks for playing!
For the code, pretty much every behavior had its own routine (for example, the screen clearing ultimate attack that every boss does at 20 seconds, or shooting in radial/line/direct patterns, or movement, or slams, etc) and then I layered those together. I wanted to make sure every boss had at least two orthogonal behaviors (so if you get used to one behavior, something else will happen to distract you), and there was a good mix of predictable patterns (I spent a lot of time on wolfram alpha tweaking sine functions) and surprising patterns that required you to learn how a boss worked.
If you want to see in more detail, I did a dev stream on my Twitch channel and the VOD is here: