In the old forums, this was a response of mine to a question regarding the level 5 boss in R-Type II.
As you know, the screen is filled with "Tetris pieces" that rotate when they come in contact with your shots or with the enemies that appear from the top and bottom of the screen. In the bottom-left corner of the screen there is a "L-shaped" piece instead of a "I-shaped" piece. (I hope this is making sense; if not, please say so and I will try to explain it better....)
Anyway, I approach this final area in the bottom-half of the screen, firing continuously. This way, if the aforementioned "L-shaped" piece is in an "upwards" position, my shots will rotate it into a "downwards" position. I then position myself directly above the "L-shaped" piece. At this point, my ship's position is such that when the lower of the two green beams is fired from the wall, it will pass directly above me.
This is a safe spot. If you continue to fire, you cannot be touched by anything, with or without Bits. You can simply time out the boss from this position.
If you are bent on the boss's destruction, then you cannot simply remain in the safe spot. You will destroy the bottom-most core, and probably the middle core, but certainly not the top-most core. In order to do that, I usually try to move to the analogous position at the top of the screen, if it exists. I say "if it exists" because the "Tetris piece" in the top-left corner of the screen is not always a "L-shaped" piece. As I recall, it is sometimes a "I-shaped" piece, and if this is the case, then I don't ever risk going up there.
One word of warning: sometimes in the final moments of my approach, the bottom-left "L-shaped" piece spontaneously rotates back up into the "upwards" position, which of course is fatal. I have not yet been able to discern why this happens, but at least it doesn't happen very often. Once the screen has stopped scrolling, if you are in the position I described, you are absolutely safe.
As an addendum to my initial post...
I think I now know why this happens. If you have two Bits and
the red laser, the bottom Bit's laser fire will sometimes cause the Tetris piece to rotate back into an "upwards" position. So if you don't have two Bits, or if you don't have the red laser, then you have nothing to worry about.
If you do
have two Bits and the red laser, then you run the risk of killing yourself if you're not careful. To make sure this doesn't happen, fire upon your approach so that the piece is in a "downwards" position. Then, stop firing altogether. Instead of placing yourself at the far-left of the screen, move forwards a little bit so that your Bit is immediately to the right of that portion of the Tetris piece that is pointing downwards. Now you can start to fire again, and you can be sure that the Tetris piece will remain motionless. In addition, this spot -- although it looks slightly more scary than the one I recommended before -- is still a safe spot, so you can time out the boss if you are unable to destroy it.