This stage is frustrating, and it's likely to make you heave your controller at the screen. But don't do that: the last thing I want is to have you damage your television.
The main difficulty in this stage is the crystal dispensers, because they constantly change the intervals at which the gems fall, and one hit will send you to your grave in the bottomless pit below. My best advice is to use Starman's Star Crash to jump safely.
The Beat Letter is in a pitfall about two thirds of the way through the stage. It's tricky to get due to being in the middle of the fall, so you'll have to get it while dodging the spikes in other areas of the corridor. It's a good idea to fall down the pit once just to get an idea of where it is and what you're dealing with. You can then kill yourself and go back for it.
Crystal Man himself is an average Robot Master. Not too simple, but not too hard either. He fires crystals that bounce off of the walls, and also throws some standard shots into the mix as well. Dodge his attacks and return fire with the Gyro Blade. Remember, you can guide the Gyro shots up or down with the control pad one time after firing, making him easier to hit in the air.