Mega Man 4/Toad Man
Toad Man's stage is filled with areas in which a heavy rain is falling constantly. When you jump, this rain will push you backwards, so be prepared to compensate for this, each time you have to jump. If you're not careful, the rains will carry you to a premature death.
Twice in the stage, you'll encounter a toad-like mini-boss. This boss has two ways of attacking - firing explosives out of its back, and launching its eyes off of its body toward our hero. Dodge these attacks, and fire at the robot's eyes when they open.
Eventually you'll reach Toad Man.
 Toad Man
After you learn this Robot Master's AI, he is arguably the easiest Robot Master of all time. Although he is weakest to Drill Bomb, don't bother using it even if you have it. Use the regular Mega Buster to rapidly fire while staying as close to him as possible without getting hit. If you shoot a few times, Toad Man will be forced to hop (he is programmed to avoid your attacks) over your head. When he lands, rapidly fire some more. He'll repeat this strategy for the entire battle, never fighting back. Just keep firing, and he won't get a chance to unleash his Rain Flush attack. If you want to use charged shots, simply cause him to jump, charge up, release it as quickly as you can, then fire a few times before he finishes releasing Rain Flush.
Victory here gets you Rain Flush and Rush Marine Adapter (useful in underwater areas).