At the beginning part of Clown Man's level, shoot a mega-ball into its mouth for an extra life.
In the first stage (after the underwater passageway), keep going to the left until you find a large health sphere. Go to it to get most of your health back.
To defeat Grenade Man, use the Thunder Claw.
To defeat Frost Man, use the Flash Bomb.
To defeat Tengu Man, use the Ice Wave.
To defeat Clown Man, use the Tornado Hold.
To defeat Aqua Man, use the Astro Crush.
To defeat Sword Man, use the Water Balloon.
To defeat Search Man, use the Flame Sword.
To defeat Astro Man, use the Homing Sniper.
After defeating the first eight Bosses, the Castle Bosses weaknesses are as follows:
Castle Boss 1: MegaBall
Defeating Dr. Wily
Castle Boss 2: Astro Crush
Castle Boss 3: Thunder Claw
Dr. Wily stage 1: Flaming Sword
Dr. Wily stage 2: Charge Dart (Get from Roll)
When you fight Dr. Wily for the first time, have the Arrow Shot equipped. This is an item that can be created from bolts. Also have Astro Crush as your Mega Power. As you begin your battle, notice the thing that spins around and does four different things (shoots about five multiple shots at you; rolls at you and returns; bounces up and down; shoots a big projectile in the air and dops down to make two smaller ones). Also, sometimes the mouth on the ship will open to fire a big laser. When this happens, fire a charged shot at the mouth. The only time that you should use the Astro Crush is when that circular thing fires the multiples shots at you; they are difficult to avoid and you will not get hurt by them if you use the power. When it rolls at you, jump slightly early. When it bounces, wait for it to bounce twice then slide under it. When it fires the big shot into the air, slide to a different location as the shot will go where you were standing. Then, jump when the two smaller shots go across the floor. If you do these things and are extremely careful, you can defeat him the first time without taking any damage. For the second battle, save the Rush Health option; all of Wily's attacks are very difficult to avoid.
Defeating the Giant Clam
When you first encounter this Boss (when playing the game from the beginning), you will not be able to do this. However, if you want to return the intro stage and defeat the Giant Clam again, the fastest and easiest way is to use the Astro Crush one time. All of its health will go away.