I'm making some progress on the first boss fight. I spent a lot of time (more than I would've liked) debugging issues with inconsistent hit detection, which I think came from Godot's raycast being used against an Area3D. It wouldn't detect it sometimes. I found a workaround and now the hit detection is 100%.
Also trying to move the shapecasts using IK + my custom stretching script for these new enemies did not work either. This was due to Godot not updating skeleton positions within PhysicsProcess. Apparently it's a complicated and unwieldy process to get those positions. I found a workaround for that as well, and it works fine.
Spontaneous enemy spawning is working now, but I still have a few more features for this fight I want to add. My plan is for Lea to take care of the enemies coming in from the front, but there are going to be threats from behind the platform as well.
Development should be a lot smoother now that it's more routine stuff that I'm familiar with. 