First thing you need to know: you shouldn't be spending any time as Blaze period. Get the timing down for how long your fireball takes to charge and how long he takes to stop flashing (being invulnerable) after taking a hit, and also where on the screen he comes back from after this. Then just time it so that you release the fireball at the earliest possible time you can hit him, and make sure you're in right front of his face when you do. Nega should never get an attack in, ever.
As for Sonic...I can't offer much help there. It's very unlikely, if not maybe impossible, for him not to do the time-consuming missile attack at least once (maybe twice). Just try to figure out how to consistently make use of the orbiting laser orbs so that you can hit him with both, one after the other, consistently (I haven't mastered this yet). The direct plasma-blob attack is easy and you should be able to hit him with that straight away every time.
I don't know if this is exactly what you wanted but that's all the help I have to offer. :[ Really, unlike this other bosses, this one is much more based on skill than how lucky you get with the attack patterns.