It took me about 6 months to master Sidearms, about the same amount of time it took me to master Sinistron. The key to victory is the Shotgun...Shotgun + Alpha = unstoppable. Once you're into Stage 4, if you lose your Shotgun then you're as good as dry toast in Ethiopia (yeah yeah, bad joke, sue me). For the bosses, here's a detailed strategy I developed (I don't know what the regular bosses are called):
Type 1 Boss (level 1): Use the 3-way to deliver a serious pelting to this ugly mofo while stopping incoming homing missles.
Type 2 Boss (level 3 and 5): Again, stick with the 3-way for good damage and bogey-killing. This guy takes a beating though. Much easier than his arcade counterpart.
Type 3 Boss (Level 6, 7, 8, and 9): Switch to the laser cannon (Mega Ballistic Laser)...you have to take this guy out FAST and the laser cannon is as strong as it gets. Watch out for his charging attack and dodge the best you can. If you get killed, run into him when you respawn for an extra dose of damage.
Type 4 Boss (Level 2): The first of the weird rotating bosses, this one shoots normal shots at you. Shotgun ftw.
Type 5 Boss (Level 4): The second of the weird rotating bosses, this one shoots lasers at you. Use the laser cannon to take it out quickly. This is the only one of the rotating bosses you fight that you have nothing available to block its shots.
Type 6 Boss (Level 9): The last of the weird rotating bosses, this one shoots homing missles at you. Three-way shot on rapidfire will put this one down quickly.
Nemesis-10 (Final Boss): Use Shotgun + Alpha or you'll be toast in mere seconds from the sheer amount of firepower this thing has. You'll also have to find one of the two safe spots on the screen where you can continuously deal damage, block shots, and not get hit all at the same time.