.....Well. That probably doesn't help very much then, does it...
Er... it's hard to outright explain how to deal with a bullet-hell pattern, really. Well, first phase: Mostly use small, short movements to deal with it... just keep slowly shifting around, at your max range. The shots that break into even more shots aren't random... that's all aimed shots; one shot from each little burst is always directly at you, the others are at angles away from you. So where all the chaos goes is dependent on where you are. Move too much and it'll just get wonkier.
Second phase: Stay far away, and start doing circles around it. You pretty much *have* to do this. Clockwise or counter-clockwise, it doesn't matter, choose one and start circling. You wont be able to turn around. It constantly re-creates that circle of green shots: never go inside of that, always stay outside. With the way the pattern works, as you're constantly rotating around the outside of the arena, the boss will effectively throw out walls of shots in your way, slow down and pass through those, and keep going, still don't turn around (and certainly don't stop). MOST of your movement here should be with the slowdown function used. If you don't pass through those little walls quickly enough, they'll distort and hit you, so you may need a burst of slightly faster movement at times.
Hope that helps without being too confusing.