Here's where your brother kicks things into high gear, and throws a REAL challenger of a stage your way.
Returning threats include the Copter Joes from Napalm Man's stage, as well as those big stomping robots you've encountered so many times already, including in the earlier Proto Man stages. The formers are easy to kill, but you'll want to avoid the latter if possible. You want to get through here in one piece!
The new obstacle this stage introduces are moving block platforms. They snake across bottomless pits, and you need to stay on them in order to cross the massive chasms. If you're far enough across a pit, you can use Rush Jet or Super Arrow to ease the burden of crossing the rest of the way on the snakey platforms, but don't run out of energy for the Rush Jet over a bottomless pit. And look out for spikes!
The third Dark Man is the most challenging one of all. With a purple body and a cannon on his shoulder, doesn't he kind of remind you of Vile from the X series? Either way, you can win this fight if you know what to do. He'll fire two kinds of shots - the expected harmful ones, and hypnotic ones that freeze you in your tracks. DO NOT, I repeat, DO NOT get hit by the ones that freeze you! Once you're frozen, you're more or less guaranteed to take damage. Use the Gyro Attack to contend with this threat, staying on the move all the while.