Which boss is giving you trouble? The final one? (big head)
The hardest thing about him is learning the attacks. Once you have that down he gets really easy, even on hard.
I usually position around mid-screen at mouth-level, then move left or right when the eye lasers bend down, or that spike from the bottom of the screen comes up. There's some warning time before the mouth-fire attack, I move down a bit when I see it.
So basically, it's just moving left and right, with occasional, down when you see fire in his mouth.
--- Even though it's been 25 years since Ken was king of the street fighting circuit, it's nothing a few good bionics can't fix. -- Street Fighter 2010