I do not have a lot of experience with the Bullet Hell genre but one of my friends helping me with this project does, he is giving me some designs for bosses that I will implement and design a level for. I am approaching finishing the implementation of the first such boss.
The boss itself simply moves back and forth across the screen, Its attack pattern is spawning 6 projectiles from specific spots and after a delay the projectiles launch towards the player. Before the projectile launches it is turning to track the player, when it is launched the tracking script is turned off and a force is applied to the object. The effect is that if the player continues to move the projectile will not hit them, but the player must be wary of being driven in to a corner and hit by the other projectiles.
As the Boss gets lower on hit points the pattern changes twice. At first the projectiles spawn above the player, in the second pattern they spawn on both sides of the screen along the edges, and the final pattern spawns along the bottom of the screen. In addition to changing patterns the delay between shoots firing also decreases and the speed of the projectiles increases slightly.
This boss may be a bit on the easy side for a bullet hell but it is the boss of level one and the other bosses will be harder.