hp for bosses

I have never, ever, been hit by a powered lower dark star, and I solo quite often.
I take it you won't be hit by the 2nd form because cannon hit hard enough to keep it KBed, but if for some reason your ship just broke? Would rapid fire keep it back even though the separate hits are so weak?

Not unless you're appled or using crazy gear - you need over 5500 min range to consistently KB 2nd form with max RF. It's better to use Bullseye or Burst Fire, anyway.

I always make sure I have my Ship before finishing the first form. I only have level 2 Ship, so it's quite likely that I'll run the 84k HP down to nothing before I kill the first form (usually I have a sliver left, and sometimes it's blinking), especially if I go in with a damaged ship. If it's blinking before the first form dies, I just finish it off with Burst Fire.

If for some reason I don't have my ship when 2nd form spawns, I just spam Bullseye.