Ah, yeah, it was probably the corrosive things. The boss should be firing a special shot that constantly chases you while attacking on it's own.
I have yet to find a way to make bullets that cant be cancelled, so that's a problem. I can make them not vanish upon impact with a target, but that doesnt stop cancellation.
Maybe there IS a way to do it, but as usual the XML document is too much of a maze for me to find anything in it. A more intrepid explorer than myself would have to go in search for that.