Thanks a ton for playing! I've been loosely planning a technical devlog post-jam to go over a lot of the lower level details, so I'll send you a link when that's up!
I wasn't planning on open sourcing the game's code, but I might open source my engine code if I get the time to clean some stuff up. Regardless, I'll definitely include that "trail" VFX in my technical devlog, it's a lot simpler than it looks! (In a nutshell I'm basically just spawning some pooled 'gameobjects' [which are just tables with x, y, color, and texture/sprite] between the start/end position of the boss, and changing the alphas of the sprites over time, but I'll go over that in more detail in the final devlog).
Glad you liked the screenshake! (as an aside, the red squares moving in the HP bar was actually a happy accident. My drawing code is clamped to the nearest pixel and my UI layer is included in screenshake, so rounding errors end up causing the red squares to move around haha!)
Thanks again for playing! I'm glad you enjoyed it!