We finally...FINALLY...do Toad Man.
Drill Man
The red turrets open and close every few seconds, shooting out a single shot while it's open. They're pretty easy for the most part. Just shoot at them when their cannons are open.
The wheel robots close themselves up and start spinning around when you're close enough to them, and they're indestructible while they're spinning. Wait until you can see the whites of their eyes before shooting at them, and use charged shots if you can.
The tubes with the triangles pointing down drop rocks. You can shoot them down if you need to, but you can just wait for them to stop dropping momentarily before passing them by. The robots that continuously spin just home in on you and can be destroyed in one hit. Nothing special. If they get the drop on you, they'll stick to the floor and shoot at you. They're still weak.
Hit the blue switches on the wall to trigger more of the level. Be careful, as there are pits that you can fall down if you're not careful. Just jump into the switch at the edge of the pit and keep going to the right as you fall, and you'll make it.
Drill Man's weakness is the Dive Missile. He burrows into the ground with the drill on his head, then he comes back up at a random location, so you'd better move around and keep jumping while he's underground. When he comes back out, he'll come at you and start firing explosive drills every few seconds. You're better off shooting him with the Dive Missile and jumping over all of the missiles, because this part is annoying as all get out. He'll also jump around, too. He'll switch it up between the whole jumping around, shooting and diving into the group at random, as he doesn't have a set pattern. Get your energy cans ready, folks.
Beating Drill Man gives you the Drill Bomb as well as the Rush Jet Adaptor.
Toad Man
Finally, Toad Man. I suggest starting here, since he's the easiest boss in the game (and probably the entire Original/Classic saga, depending on what you want to call it) to beat with the regular arm cannon.
The rain in this area pushes you back, so you'd better be on the very edge of a platform when you jump to another one.
The bird robots have three smaller clones behind them that home in on you when you're close enough. This is the best introduction to the charge shot in my opinion, as firing one at the big bird takes out the three behind it as well. Sheesh, this thing would have made the second and third games easy as anything.
The robots that come down on parasols can also be taken down rather easily with a charged shot. Same thing with the robot rats as well as the little bubble robots that can stick to both the floor and ceiling as they chase after you. Once again...best introduction to a new feature ever by having you finding out that using it makes the whole stage really easy except for the jumps.
The water on the ground from the waterfalls will push you around according to where they're moving. If they're moving left, you're moving left. 'Nuff said.
Shoot the snails in the eyes when they're open, and dodge the shots that it shoots at you like a catapult. Charged shots make this easier, as there's a chance that you won't even have to deal with its second attack - the snail's eyes coming right at your present location. The second snail battle has a waterfall involved, and the water lands almost dead center on the platform with a pit on both sides of it, so you'll have to be on the move constantly for the second one.
The fish are easier to avoid here than on Dive Man's stage. Just be careful with the platform directly under another one (there's only one instance here), since you can bump your head and land right into the spikes below.
Toad Man is really, really easy. You could beat him with the Drill Bomb...but screw it. Hit him when he's jumping around with regular shots. Move around and shoot him till he stops jumping and stands still. This will usually happen if you start shooting at him from a fixed point. When he stands there, he's about to do some sort of hula dance where he activates his completely unavoidable Rain Flush attack. There's a trick to make him not do it. Shoot him when he's doing the little hula thing just before the attack launches, and you'll instantly cancel it out. He's that easy, as he doesn't move until you do and he fires the weapon if you're not moving.
Beating Toad Man gives you the Rain Flush and Rush Marine Adaptor.
Bright Man
The green guys that walk around shoot out fireworks that light up the room, while the red robots with the light underneath them that fire a shot that splits into five projectiles that fan out. Destroying a red robot will darken the room, since they're red light bulb robots. Destroying a green guy afterward lights up the area again.
The totem poles just spit projectiles at you from the second totem, while the robot crickets act as platforms that help you ride across the massive spike pits. Just stand on them and they'll start moving.
The green robots that hang on the ceiling and come down at you before spinning around like a top in your direction can be easily rid of with a charged shot.
After that, go down and head to the right to get an extra life and an energy can. The red platforms fall down when they reach the end of the track, but the green ones actually go back. You'll see these a little later in the stage. Use Rush Coil if you want to keep your extra life. It's up to you.